A．help mariners.
B．improve ship design and sailing method.
C．study astronomy and mathematics.
D．improve his own skill as a sailor.
参考答案B
解析:
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A．make longer deep-sea voyages.
B．travel faster than those in use at that time.
C．explore the coastline of Portugal.
D．carry larger crews and more cargo than existing ones.
A．Making oceanic exploration possible.
B．Improving the compass.
C．Founding a school for mariners.
D．Inventing the clipper ship.