A: What’s the matter, Sara? You wear a sad look on your face.

B: Oh, nothing much. As a matter of fact, I _______ of my friends back home.

A．just thought

B．have just been thinking

C．was just thinking

D．have just thought

刚玉砖主要化学成份为（）。

A.Al₂O₂

B.SiO₂

C.MgO

D.SiC