二、单项填空（本题有15小题，每小题1分，共15分)
请从A、B、C、D四个选项中选出可以填入空白处的最佳选项。
16. — Which one is your mother?
— ______one in a purple skirt under the big tree.
A. A B. An C. The D. /
17. —Who is the best friend of ______ at school?
—I think Helen is. We often help each other.
A. mine B. his C. yours D. hers
