______ in his studies, he didn't know that all the others had left.

A．Buried

B．Burying

C．Having buried

D．Bury

（）不宜长时间关井，为延长胶芯寿命，防喷演习时不进行封零。

A、半封闸板防喷器

B、全封闸板防喷器

C、环形防喷器

D、剪切闸板防喷器