三级基坑为开挖深度小于( )，且周围环境无特别要求时的基坑。
A．3m
B．5m
C．7m
D．10m
参考答案：C
短暂性脑缺血发作最常见的病因是( )。
A．糖尿病B．动脉粥样硬化C．风湿性心瓣膜病D．高血压和动脉粥样硬化E．高脂血症
It's difficult _____ your sound because the noise is too high. [ ]
A. to listen
B. to know
C. to learn
D. to hear
