Wet and Dry Periods on the Great Plains
Paragraph 1: By and large, the climate of the North American prairie is unreliable. As rival air masses interact with each other over the plains, they keep the atmosphere in a state of more-or-less constant flux, so that the weather oscillates from extreme to extreme. But there are also times when the climate system seems to get stuck — wet spells (periods) for example, when the rain refuses to stop; or dry spells of months — or years — when the clouds seem dry as parchment and the air fills with dust.
1. The word “rival” in the passage is closest in meaning to
O unstable
O competing
O rising
O growing
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