She spent the whole evening talking about the things and persons ______ none of us has ever heard of. [ ]

A. which

B. who

C. whom

D. that

在层数较多的建筑物内，为了防止底层卫生器具因立管底部出现过大正压等原因而造成污水外溢现象，底层的生活污水管道应考虑采取单独排出方式。