问题补充：
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
答案：
A. at the expense of
B. at the risk of
C. in the way of
(答案→)D. at the mercy of
解析： 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。
