千斤项按结构形式和工作原理可分齿条式千斤顶、螺旋式千斤顶和（）式千斤顶三种。
参考答案：液压
A huge amount of oil was spilt on the East Sea, the caused of ______ are still under investigation. [ ]
A. it
B. that
C. which
D. whom
根据《保险法》的规定，保险合同成立的时间是( )
A．受要约人承诺生效之时
B．书面保险合同签订之时
C．保险单签发之时
D．保险费缴纳之时
