传声器的灵敏度是指传声器的输出端的输出电压与输入端的（）之比。
A、声压级
B、声压
C、响度级
D、响度
参考答案：B
设集合A={2，3，a2+2a-3}，集合B={|a+3|，2 }，已知5∈A，且5∉B．求a的值．
Mr. Wang made up his mind to devote all he could ____ his oral English before going abroad.
A．improve
B．to improve
C．improving
D．to improving
