问题补充：
we know, the air quality in the city has improved over the past two years.
A. That
B. As
C. What
D. It
答案：
A. That
(答案→)B. As
C. What
D. It
解析：句意：正如我们知道，这个城市的空气质量在过去的两年间有了改善。As。正如；类似的表达还可以是：What we know is that the air quality in the city has improved over the past two years. 或者 It is known that the air quality in the city has improved over the past two years. 故选B。
考点: 考查代词的用法。
