Hi YU, maybe u can think the question this way.
On the one hand, according to your explanation, u think the 1st paragraph puts forward 2 "benefits "about the elevated CO2 level, which I donnot agree, cause benefit means we do get something salutary from it, and the whole passage argues that the increasing level of CO2 does not result in the agricultural abundance and amelioration of global warming in actuality
. And the pivot at the beginning of paragraph 2 indicates what the author's gonna do is to rebut "some research" mentioned above. So I think we can conclude that the "benefit" here is a misconception used by some researchers.
On the other hand, if there was nothing wrong with "benefit" , I mean there do exist 2 benefits about the enhancing CO2 level, the answer (b) just mentions only one of the two aspects: "contradictory findings about the benefits of increased levels of CO2 on
agricultural productivity" ,
so it's not the correct one due to incompleteness .
Let me know if it helps.