English teachers are supposed ______their students a test in class, even after class, ______better teaching results.

A．to give; ensuring

B．giving; to ensure

C．to give; to ensure

D．giving; ensuring

下列说法正确的是[ ]

A．摩尔是一种国际基本物理量

B．标准状况下气体摩尔体积约为22．4L

C．1mol氧的质量为16g

D．在同温同压下，相同体积的任何气体单质所含分子数相同