单项选择题

Which of the following titles is most appropriate to the passage

A．Polarity Reversal: A Fantastic Phenomenon of Nature.
B．Measurement of the Earth's Magnetic-Field Intensity.
C．Formation of the Two Poles of the Earth.
D．A New Approach to the Study or Geophysics.

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单项选择题

Although accidents do happen, skiers will be reasonably safe if ______.

A．they stay on the officially approved slopes
B．they ski only for pleasure
C．they ski only at resorts
D．they choose less crowded ski slopes

单项选择题

Each winter holiday makers in the mountains come face to face with death because ______.

A．they fail to realize how dangerous avalanches can be
B．they are not expert enough at skiing on the fixed runs
C．they are trying to avoid areas where avalanches happen
D．they ski great distances down the mountainsides