Read the article and fill in the missing parts of sentences. One of them is extra.
a) exist in only one form;
b) the photon and gluon look the same;
c) whether anti-hydrogen behaves;
d) a vast amount of matter and antimatter;
e) positive energy was expected;
f) to reveal the antiproton;
g) which are reversed;
h) the track of a positively charged particle.
THE MYSTERY OF ANTIMATTER
 Fictional spaceships are often powered by ‘antimatter drives’, yet antimatter itself is real and has even been made artificially on Earth. The history of antimatter’s discovery began in 1928 when British physicist Paul Dirac saw that his equation for the electron offered the possibility that electrons could have negative as well as positive energy. Dirac had two ways of solving his problem: [1__], associated with a normal electron, but negative energy made no sense. But rather than ignore this confusing term, Dirac suggested that such particles might actually exist. This complementary state of matter is antimatter.
 The hunt for antimatter began quickly. In 1932 Carl Anderson confirmed the existence of positrons experimentally. He was following the tracks of showers of particles produced by cosmic rays. He saw [2__] with the electron’s mass, positron. So antimatter was no longer just an abstract idea but real.
 It took another two decades before the next antiparticle, the antiproton, was detected. Physicists built new particle-accelerating machines that used magnetic fields to increase the speeds of particles travelling through them. Such powerful beams of speeding protons produced enough energy [3__] in 1955. Soon afterwards, the antineutron was found.
 With the antimatter equivalent building blocks in place, was it possible to build an anti-atom, or at least an anti-nucleus? The answer was yes. A heavy hydrogen (deuterium) anti-nucleus (an anti-deuterium), containing an antiproton and antineutron, was created by scientists at CERN in Europe and Brookhaven Laboratory in America. Tagging on a positron to an antiproton to make a hydrogen anti-atom (anti-hydrogen) took a little longer, but it was achieved in 1995. Today experimenters are testing [4__] in the same way as normal hydrogen.
 On Earth, physicists can create antimatter in particle accelerators. When the beams of particles meet, they annihilate each other in a flash of pure energy. Mass is converted to energy according to Einstein’s E=mc2. So if you met your antimatter twin, it might not be such a good idea to throw arms around them.(A) If antimatter were spread across the universe, these annihilation episodes would be occurring all the time. (B) Matter and antimatter would gradually destroy each other in little explosions, mopping each other up.(C) In fact normal matter is the only widespread form of particle we see, by a very large margin.
 Like all mirror images, particles and their antiparticles are related by different kinds of symmetry. One is time. Because of their negative energy, antiparticles are equivalent mathematically to normal particles moving backwards in time. So a positron can be thought of an electron travelling from future to past. The next symmetry involves charges and other quantum properties, [5__], and is known as ‘charge conjugation’. A third symmetry regards motion through space. Returning to Mach’s principle, motions are generally unaffected if we change the direction of coordinates marking out the grid of space. A particle moving left to right looks the same as one moving right to left, or is unchanged whether spinning clockwise or anticlockwise. This ‘parity’ symmetry is true of most particles, but there are a few for which it does not always hold. Neutrinos [6__], as a left-handed neutrino, spinning one direction; there is no such thing as a right-handed neutrino. The converse is true for antineutrinos which are all right-handed. So parity symmetry can sometimes be broken, although a combination of charge conjugation and parity is conserved, called charge-parity or CP symmetry for short.
 Just as chemists find that some molecules prefer to exist in one version, as a left-handed or right-handed structure, it is a major puzzle why the universe contains mostly matter and not antimatter. A tiny fraction – less than 0.01% - of the stuff in the universe is made of antimatter. But the universe also contains forms of energy, including a lot of photons. So it is possible that [7__] was created in the big bang, but then most of it annihilated shortly after. Only the tip of the iceberg now remains. A minuscule imbalance in favor of matter would be enough to explain its dominance now. To do this, only 1 in every 1010 matter particles needed to survive a split second after the big bang, the remainder being annihilated. The leftover matter was likely preserved via a slight asymmetry from CP symmetry violation.
Look at three lettered spaces in the text (A),(B),(C) that indicate where the following sentence can be added to the passage. Where would the sentence best fit?
Because we don’t see this, there cannot be much antimatter around.
4. Read the text again and find the words that mean the same as the following definitions.
a. Similarity under reflection or rotation or re-scaling;
b. A line of light, electric waves or particles;
c. Combination of some things;
d. Either of two numbers or letters used to fix the position of a point on a map or graph;
e. A statement showing that two amounts or values are equal;
f. The act or process of moving or the way something moves;
g. The state of being equal;
h. The rate at which sb/sth moves or travels;
i. The path or direction that sb/sth is moving in;
j. To destroy sb/sth completely.