A:4019
B:4020
C:4021
D:4022
计算2m2m+n-m-nn+2m的结果是( )
—________ is your Maths teacher?
—The man ________ a white shirt.
假期里我学会了扫地。( )
在化学方程式:Cu+2H2SO4(浓)═CuSO4+X↑+2H2O中,X的化学式是( )