在继电电路中研究的主要问题是电路的结构问题。
参考答案：错
In big cities like London, Paris, New York, you can see street musicians _____ all kinds
of music such as Rock'n'Roll, Jazz, the Blues in the streets. [ ]
A. perform
B. make
C. act
D. do
属于大气中的二次污染物是
A．CO
B．SO2
C．CO2
D．H2S
E．SO3
