under the tree was a charming girl aged about seventeen and eighteen.A. SatB. SittingC. T

问题补充：

under the tree was a charming girl aged about seventeen and eighteen.

A. Sat

B. Sitting

C. To sit

D. Sit

答案：

A. Sat

(答案→)B. Sitting

C. To sit

D. Sit

解析：考查倒装。句意：正坐在树下的是一位年约十七八岁的可爱女孩。表语放在句首加强语气时，其结构为：v-ing/v-ed+介词（短语）/副词+be+主语+其他成分。根据句意，选择句意及倒装结构选择答案B。