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

卵子受精后第（）日分裂成16个细胞组成的实心团块称为桑葚胚，于第（）日早期囊胚进入子宫腔，约在受精后第（）日囊胚开始着床

A.2，3，4～5

B.3，4，6～7

C.4，5，7～8

D.5，6，8～9

E.6，7，8～9