"Did Not Exceed" in C

Hi LSAT Max! I'm still a little confused with this stimulus and correct answer choice. I know that the cause is the experimenter concluding that fusion has been achieved because the effect was a measurable amount of h-4 gas found in the air of the chamber. When I first picked C, I thought that "did not exceed" meant that there wasn't enough h-4 gas which is why it weakens. After reading the explanation and answer choice C about five more times, however, I think "did not exceed" references to the fact that the h-4 gas was either the same amount or less than what is typically found in ordinary air. Since there's no difference in h-4 gas between the experiment and generally with ordinary air, does that lead us to believe that the presence of h-4 gas isn't enough to prove that fusion has been achieved? I hope I didn't lose you there with my question. Any clarifications would be greatly appreciated. Many thanks in advance!