When asked to explain _____ he did to make his students so fascinated with his lessons, the teacher paused and thought deeply.

A．what was it that

B．that what it was

C．what it was that

D．what was it

下列表示粉尘光学特性的是（）。

A、反射能力

B、透光程度

C、光强衰减程度

D、照射能力