单项选择题

Directions: In this section,there are 10 incomplete sentence& You are required to complete each one by deciding on the most appropriate word or words from the 4 choices marked. Then you should mark the corresponding letter on the Answer Sheet with a single line through the center. The old man has two daughters, ______ are teachers.

A. both of them
B. both of whom
C. both who
D. they both