The air quality in the city, is shown in the report, has improved over the past two months

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A. that
B. it
C. as
D. what
解析：本句难点是把定语从句分割出来放在句首就很明了了，把句子转换成    is shown in the report, the air quality in the city has improved over the past two months.就能直接看到非限制性定语从句引导词放句首指的是下文所提到的一句话用as引导“正如在 所写的那样”。