China Business Daily reported the 60% discounts on some Beijing-shanghai flights have now slipped back to 20% _______ one day in advance.
A．when booksB．when to bookC．when bookingD．when booked
D

試題分析：考查狀語從句的省略。當狀語從句的主語與主句的主語一致，且狀語從句中有系動詞be的時候，可以把狀語從句的主語和be動詞一起省略。本題在when和booked之間省略了it is；句意：如果提前一天預訂，從北京到上海的機票折扣從百分之60下降到百分之20.故D正確。
點評：當狀語從句的主語與主句的主語一致，且狀語從句中有系動詞be的時候，可以把狀語從句的主語和be動詞一起省略。如果狀語從句中有it is的時候，it is 也可以省略。
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