——Fifty dollars for such a T-shirt! You _______ be joking!——I'm serious.It's made of silk.
A:must
B:need
C:may
D:can
His d____ made us realize that our life is easy to lose.
洗涤在生活、生产中不可缺少.下列洗涤方法中利用了乳化原理的是( )
在△ABC中,已知AB→·AC→=AB→·CB→=1则AB→的值为 ( )