The voyages of travellers before the 17^th 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

答案及解析： D

短语辨析. A.以……为代价； B.冒着……的危险 C. 妨碍；关于……方面 D. 受……支配。句意：十七世纪前的旅行者的航行告诉我们尽管他们没有现代助航设备，他们也不受大海支配。故选D。