表达正在进行的动作 — 英语选择题
题目
周末的公园里很热闹。Tom 正在给远处的朋友打电话介绍："Listen! The birds are singing. Look! My sister ______ a kite on the grass, and the twins ______ swimming in the lake." 空格处依次应填入的正确选项是（ ）
A. is fly; is swimming
B. is flying; are swimming
C. are flying; is swimming
D. is flying; is swimming