For this question, I was stuck between A and E, eventually choosing E.
I'm trying to show why A is the better answer, but I'm a little unclear. E doesn't appear right because of what it states: effective mechanism to prevent atmospheric gases, when only one gas, oxygen, is stated as bending competitively.
A, while correctly stating that the productivity is due to the C-4 process, only mentions maize, while in the first paragraph, and subsequent paragraphs, discuss the process in a more general context--for not just maize, but for these more productive plants in general.
So I guess I'm confused on A due to the scope. Both A and E seem to have a problem.