题目

I usually_______ books in the morning. A.look       B.read       C.watch       D.see 答案：B1．已知|$\overrightarrow{a}$|=2与|$\overrightarrow{b}$|=4，在下列条件下求$\overrightarrow{a}$•$\overrightarrow{b}$：（1）$\overrightarrow{a}$∥$\overrightarrow{b}$；（2）$\overrightarrow{a}$⊥$\overrightarrow{b}$．