You can never know ________ when she received our nice birthday presents.

A．how Mary was excited

B．how excited Mary was.

C．How excited was Mary

D．Mary was how excited

下列各组金属最合适用氢气或CO把它们从化合物中还原出来的是（   ）

A．Ca、Mg

B．Al、Fe

C．Fe、Cu

D．Hg、Ag