单项选择题

This passage is mainly about ______.

A．the beginnings of oceanography
B．the laying of the first undersea cable
C．the investigation of ocean depths
D．the early intercontinental communications

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单项选择题

The aim of the voyages Maury was responsible for in the 1840 was ______.

A．to make some sounding experiments in the oceans
B．to collect sample of sea plants and animals
C．to estimate the length of cable that was needed
D．to measure the depths of the two oceans

单项选择题

People tend to ignore the delayed effects of exposure to chemicals because

A．limited exposure to them does little harm to people's health
B．the present is more important for them than the future
C．the danger does not become apparent immediately
D．humans are capable of withstanding small amounts of poisoning