—What do you think of the two students    ________     to take part in the competition?

—They are hardworking as well as intelligent.

A．selecting

B．selected

C．having selected

D．to select

下列物质属于纯净物的是（    ）

A．洁净的空气

B．精制的食盐水

C．冰与水混合

D．高锰酸钾加热完全反应后的剩余物