单选题 Nuclear fusion is a process whereby the nuclei of atoms are joined, or "fused," and in which energy is released. One of the by-products of fusion is helium-4 gas. A recent fusion experiment was conducted using "heavy" water contained in a sealed flask. The flask was, in turn, contained in an air-filled chamber designed to eliminate extraneous vibration. After the experiment, a measurable amount of helium-4 gas was found in the air of the chamber. The experimenters cited this evidence in support of their conclusion that fusion had been achieved.
Which one of the following, if tree, would cast doubt on the experimenters' conclusion?

A、 Helium 4 was not the only gas found in the experiment chamber.
B、 When fusion is achieved, it normally produces several byproducts, including tritium and gamma rays.
C、 The amount of helium 4 found in the chamber's air did not exceed the amount of helium 4 that is found in ordinary air.
D、 Helium 4 gas rapidly breaks down, forming ordinary helium gas after a few hours.
E、 (E) Nuclear fusion reactions are characterized by the release of large amounts of heat.

【正确答案】 C

【答案解析】[解析] 本题为Weaken结论，而本题是由一个实验的结果而得出了一个结论，我们可以Weaken前提。(C)选项表明氦-4的量并没有超过普通空气里的氦气量，从而Weaken了上面暗含的前提。(A)、(B)均与段落中“one of the by-product of fusion is helium gas”吻合，不可能为反对，而(D)、(E)均为无关选项。