A:a和b
B:b和a
C:b和c
D:c和b
The mother often worries her son's study.
—They went to the hospital to______________ the sick kids.
关于镀铜和电解精炼铜,下列说法中正确的是( )
较大会场的座位都呈阶梯形状的原因是为了____ .