Answer: E

The scientists' argument contains a slight scope shift. The mayor argues that the seaweed farms are responsible "almost exclusively" for a decline in emissions. The scientists reply that seaweed farms "can only absorb" half the amount that the mayor attributes to them. But what if seaweed farms had some other benefit besides simply absorbing emissions? The mayor doesn't claim that the twenty percent reduction is due simply to absorption.

To weaken the argument, expose and contradict the assumption that absorption is the only way for seaweed farms to reduce emissions. Choice (E) is correct--if seaweed plants can be used as a low-emissions fuel source, they do in fact have more benefit than their emissions absorption.