The voyages of travellers before the 17th century show that they were not ________ the sea even though

they didn't have modern navigational aids.   [ ]

A. at the expense of    

B. at the risk of      

C. in the way of    

D. at the mercy of  

一个质数的3倍与另一个质数的2倍之和等于20，那么这两个质数的和是( )。

A．9
B．8
C．7
D．6