Mary was only too ready to help others, seldom, _____, refusing them when they turned to her. [ ]

A. if never

B. if not

C. if any

D. if ever

要使自凝树脂在常温下就能聚合，需要有（）。

A.叔胺等促进剂

B.BPO等引发剂

C.阻聚剂

D.紫外线吸收剂

E.单体