The sudents were silent as they saw the ___ look on their teacher’s face.

A．fright

B．frightening

C．frightened

D．both B and C

安装活塞环时，要注意环的切槽方向（），或者按照所修车型维修手册安装。

A、活塞环的内边缘切槽的一面应向上

B、活塞环的外边缘切槽的一面应向上

C、活塞环的内边缘切槽的一面应向下

D、无所谓