The answer choice isn't saying the thermometer is not 1 degree. The conclusion is saying if the thermometer says 1 degree, it is within a margin of error of 1 degree. The answer choice is saying, "sure the thermometer might be reading 1 degree, but there is such a variation in temperatures between the height of the two plants, that we cannot assume the margin of error is within 1 degree".

Yes, I am as well.

A little late, but after having the most difficulty with this problem compared to other MC/MP questions, I think I understand why C is the answer, not A. While A looks like it is a perfectly reasonable answer, we are not analyzing arguments for their adequacy. While all context points to the conclusion being A, the author is really trying to tell us that ET life will never be found. This is especially tricky with the odd wording of the author's conclusion.