单项选择题

We can infer that when Harrison succeeded in making his clock he was ______.

A．in his eighties
B．100 years old
C．twenty years old
D．a middle-aged man
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单项选择题
In the fifth paragraph, the underlined word it refers to ______.
A．the earth's magnetic field
B．the sun
C．the built-in compass
D．the special way
单项选择题
Which is not true according to his passage
A．Scientists know the answer to the question how a pigeon tells direction.
B．Scientists are trying to answer the question how a pigeon finds its way home.
C．A pigeon can use some special way to sense where its home is.
D．By using the sun a pigeon can tell directions, rain or shine.
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