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题型：单选题 题类：真题 难易度：普通
—Sweeping robots _________ more and more widely today. Why not buy one?
The air is{#blank#}1{#/blank#} {#blank#}2{#/blank#}so we could {#blank#}3{#/blank#} {#blank#}4{#/blank#}see blue skies any more.
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