— I wonder if this smart phone is Mary's.— It _______ belong to her. _______ is totally different from this one.
A: mustn't; Her
B: can't; Her
C: can't; Hers
D:may; Hers
若a→=(3,-1),b→=(-3,2),则a→·b→=____
关于声现象,下列说法正确的是( )