四、单项填空(共15小题，计l5分)
从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
（ ）16. It takes about a quarter of_________hour to walk into town.
A. a B. an C. the D. 不填
（ ）17. As there were not enough chairs in the park, they had to sit on the_________.
A. ground B. earth C. land D. field
