位于冠状沟后部的结构是（）
A．三尖瓣
B．二尖瓣
C．冠状窦
D．窦房结
E．房室结
参考答案：C
The computer system ____ suddenly while he was searching for information on the Internet.
A．broke out
B．broke down
C．broke up
D．broke in
