David's younger brother is（）engineer.
A.a
B.an
C.the
参考答案：B
Even if the treatment ________, there is still no magic pill for patients in the late stages of AIDs.
A．does
B．uses
C．works
D．helps
直流电动机除极小容量外，不允许（）启动。
A、降压
B、全压
C、电枢回路串电阻
D、降低电枢电压
