同步发电机独立运行时，调节原动机油门（）改变频率；调节励磁电流（）改变电压。
A.可以；可以
B.可以；不能
C.不能；可以
D.不能；不能
参考答案：A
Unfortunately, it was in Yushu County, ______ a 7.1-magnitude earthquake happened on April 14, 2010, ______ a large number of students were buried inside damaged schools.
A．which; when
B．that; that
C．where; that
D．where; which
内痔分期的主要依据是：（）
A.脱出
B.疼痛
C.便血
D.患病时间
E.以上均不对
