The passage tells us that because the researchers found something that was a byproduct of fusion, they concluded that they found fusion. This is clearly flawed. It's possible that some other thing caused the byproduct other than fusion.
C gets to this point. It indicates that the amount of the byproduct in the experiment was the same as it would have been if the experiment had not been performed. This shows that there is something else that can lead to finding the byproduct (it is found in normal air) and that since there was no more than normal, there is no evidence for fusion.
D just doesn't do anything to the argument. So what if it decays fast? That doesn't seem to impact it to me.
