采用不通过血脑屏障显像剂的脑静态显像中脑梗死阳性发现多见于起病后（）
A.第1天内
B.第1周内
C.第2～4周
D.第5周后
E.第6周后
参考答案：C
The boys even have no time for their homework _______ the basketball match.[ ]
A. because of
B. because
C. as
D. since
UPS一般标示要求供电频率是多少（）
A.60Hz
B.55Hz
C.50Hz
D.45Hz
