投保人在合同订立前、订立时及在合同有效期内，应对已知的危险和标的有关的实质性重要事实向保险人作真实陈述。投保人履行的该义务是（）

A.危险增加通知义务

B.如实告

C.提供单证义务

D.及时通知义务

I’m not sure whether be has finished writing the book, but he________ it last month.

A．worked at

B．was working on

C．worked on

D．was working at