——Your mother's never seen a 3D film at the cinema, has she?—— ____, but only once.
A:Yes, she has
B:No, she hasn't
C:Yes, she is
D:No, she isn't
物质X符合下列转化关系:H2SO4 x MgSO4,则X不可能是( )
小于180°的角叫做钝角.