液压锤的能量传递效率比柴油锤（）。
A、高
B、低
C、一样
D、不能确定
参考答案：A
Why will the man receive a discount
A．He is buying in bulk.
B．He works for the company.
C．He is a frequent customer.
D．He has discount coupons.
关系R(A，B)和S(B，C)中分别有10个和15个元组，属性B是R的主码，则R
S中元组数目的范围是
S中元组数目的范围是