题目

     Sarah and Janet have been friends since they 31._______ (start) school. They do everything together: study, read, watch TV, surf the Internet, play 32._______ piano and listen to music. Anyone 33._______ has a problem can ask the other 34._______ help. When Sarah was in the 35_______ (five) grade, some of the other students made fun of 36._______ (she) because she was shy. Janet told the students to stop 37._______ (make) fun of Sarah, and she helped Sarah overcome her shyness. In the sixth grade, Janet had 38._______ (difficult) in her maths class. She studied hard, 39.______已知等差数列{an}中，Sn是它的前n项和．若S16>0，且S17<0，则当Sn最大时n的值为(　　)A．8B．9C．10D．16