His sister enjoys _______ tennis in her free time.
A:playing
B:plays
C:play
在平行六面体ABCD﹣A′B′C′D′中,若AC'→=xAB→+2yBC→+3zC'C→,则x+y+z等于( )
(I)求证: MA⊥MB ;
(II)求 △MAB 面积的最小值.