問題詳情

   People in cold climates are used to road signs warning that “bridge freezes beforeroad.” At first, it may not seem to make sense. The air temperature feels the same if you’restanding on a bridge or a road. However, bridges do freeze before roads. (16) is thisthe case?    First, cold air surrounds the surface of a bridge from above and below. This meansthat bridges lose heat from both (17) , so they will ice rapidly as soon as thetemperature falls to the freezing point. (18) the road surface also loses heat tothe air, the ground below roads helps to trap in heat and keep roads from getting icy.    Another reason is because bridges are usually made of steel and concrete.(19) conduct heat very well. Any heat a bridge manages to trap will be transferredto its surface quickly, where it will be lost to the air around it. Roads are made ofasphalt, which does not conduct heat very well. The heat trapped within and below aroad will stay (20) , increasing the time it takes a road to freeze.
16.
(A) Why
(B) When
(C) What
(D) Where

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