问题
选择题
---We’ve seen many more days of blue skies recently.
---True. But back in 1998, ________ Beijing started its “Defending the Blue Sky” program, the city only ________ about 100 days a year.
A．which, sees
B．that, saw
C．when, saw
D．and, seeing
答案
答案：C
回到1998年，那时北京开始了保卫蓝色天空项目，这个城市大概只有一百天的晴空。考查非限制性定语从句，根据1998年，应为过去式。