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1.Whereas when we are asleep, they work very, very hard at removing all the waste that builds up when we are awake.
build up (交通)阻塞，(交通)越来越拥挤，(车辆)拥挤起来；(用砖等)堵塞(门、窗等)：
During the summer holidays,traffic builds up along the roads to the coast.
每到暑假，在去海滨的路上车辆就拥挤起来。
The doorway in the west side was built up by the neighbour's house.
西面的门口被邻居的房子阻塞了。
