A: (451+49)﹣51
B: 451﹣49+51
C: 451﹣(51+49)
从根本上讲,元朝能够统一中国并维持近百年的原因是( )
In the south there are storms ____ ____ ____ ____ in summer.
—When do you usually eat __________?
—At a quarter to seven in the evening.
在如图所示的电路中,由于某一电阻发生短路或断路,使A灯变暗,B灯变亮,则故障可能是( )