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 true, would cast doubt on the experimenters' conclusion? ['Helium-4 gas rapidly breaks down, forming ordinary helium gas after a few hours.', 'When fusion is achieved, it normally produces several by-products, including tritium and gamma rays.', 'Nuclear fusion reactions are characterized by the release of large amounts of heat.', "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."]