题干
There was a loud cry from the backstage immediately after the concert ended, _______?
A：wasn't there
B：was there
C：didn't it
D：did it
答案(点此获取答案解析）
A
There was a loud cry from the backstage immediately after the concert ended, _______?
A：wasn't there
B：was there
C：didn't it
D：did it
角的光滑平行的金属轨道，上端接有可变电阻R，下端足够长，空间有垂直轨道平面的匀强磁场B，一根质量为m的金属杆MN从轨道上由静止滑下，经过足够长的时间后，流经金属棒的电流达到最大，金属杆做匀速运动，金属杆和轨道电阻均不计。则（ ）
