The professor _______ to walk out of the office when one of his students came in.

A．was about to

B．was going to

C．would

D．was to

已知变量X和Y的协方差为－50，X的方差为170，Y的方差为20，其相关系数为（）。

A.0.86

B.―0.86

C.0.01

D.-0.01