The scientist has made another discovery,       I believe is of great importance.

A. that            B./          C. which         D. why

The factory was built in a secret place, around       high mountains.

A. which was         B. it was         C. which were        D. them were

对于非限制性定语从句考查中句中出现逗号的情况，首先看逗号旁是否有连词，没有连词就意味着逗号前后内容一定有一部分是完整的句子，另一部分是修饰成分或非限制性定语从句。第一道题显然逗号后为非限制性定语从句，对discovery进行修饰，选which；第二道题逗号后为非限制性定语从句，介词around前置，句序还原为high mountains …around which(a secret place)，be动词跟mountains一致为复数。