Multiple choice

Every element on the periodic chart is radioactive, though the more stable elements have half-lives which are thousands and thousands of years long. When an atom decays, it splits into two or more smaller atoms. Even considering the fusion taking place inside of stars, there is only a negligible tendency for smaller atoms to transmute into larger ones. Thus, the ratio of lighter to heavier atoms in the universe is increasing at a measurable rate. Which of the following sentences provides the most critical continuation of this paragraph?

  1. Without radioactive decay of atoms, there could be no solar combustion and no life as we know it.

  2. Therefore, it is imperative that scientists begin developing ways to reverse the trend and restore the proper balance between the lighter and the heavier elements.

  3. Consequently, it is possible to use a shifting ratio of light to heavy atoms to calculate the age of the universe.

  4. Therefore, there are now more light elements in the universe than heavy ones.

  5. As a result, the fusion taking place inside stars has to produce enough atoms of the heavy elements to offset the radioactive decay of large atoms elsewhere in the universe.
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C Correct answer
Explanation

(3) The last sentence of the paragraph is very important. It tells us that the proportion of light atoms in the universe is increasing (because heavy ones decay into light ones, but the reverse process does not occur) and that this trend can be measured. By extrapolation back into time on the basis of present trends, scientists can find out when it all began.

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