下列哪项是前瞻性队列研究的优点（）

A.适合于少见病研究

B.可用于研究多种暴露与一种疾病的关系

C.可直接计算发病率

D.所需样本较小

E.省时、省力

(2013·四川省南充市二模) ________ him not to do so, he wouldn't have made such a serious mistake.

A．If I persuaded

B．Did I persuade

C．Had I persuaded

D．If I should persuade