| The safety of ships at sea does not
depend only on the arrangements made in the ships themselves. Dangerous coasts
and rocks may be seen in the daytime, (51) they are a threat
to safe navigation at night. Lighthouses are therefore erected at such places
to (52) sailors of the danger. Modern lighthouses are put up
at all dangerous places near the courses (53) by ships. On
some rocky coasts several lighthouses may be built not far from each other,
and (54) the sailors on a ship may know which is which, they
have (55) arrangements of lights. One lighthouse will
give a single flash at regular (56) , another may give two
flashes; a third may give two followed by one; and so on. (57)
watching the lights the navigator is able to decide exactly where he
is, and he can set his (58) with confidence. In A. however B. and C. but D. or [填空题]The sea level rise is mainly caused by______ triggered by unchecked greenhouse gas emissions.
[简答题]Arctic Sea
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