题目

 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：D有14瓶同样的木糖醇,其中13瓶的质量相同,另有一瓶少了几颗。如果用天平称,至少称（____）次就能保证找出较轻的这瓶来。