问题

Thevoyagesoftravelersbeforethe17thcenturyshowthattheywerenot____theseaeventhoughtheydidn’thavemodernnavigationalaide.[ ]
A:attheexpenseof
B:attheriskof
C:inthewayof
D:atthemercyof
B:attheriskof
C:inthewayof
D:atthemercyof
正确答案是: