单项选择题

When Harrison's clock was first made,______.

A．it could tell the time at sea
B．it could tell sailors where they were
C．both A and B
D．every country in the world set the time by it besides A and B

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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

Pigeons have been used as messengers because ______.

A．they have a built-in compass
B．they can cover 500 miles in a single day
C．they have special ability to tell directions
D．they can find their way home