921015（1992－10－15）Zeros修改标题，见谅

15.In general, the simpler plants appeared on the Earth before those ---.

(A) are structurally complicated

(B) more complicated structure

(C) have a complicated structure

(D) their structure is complicated

答案是B，为什么A C不行呢？before是连词吧，后面应该有主谓宾，怎样分辨它是连词还是介词呢

句中simpler为比较级，后文应出现与之相比较的内容，即另一个比较级的形式，故选(B).