Who is the girl _____ a letter there? A writing B written C

題目：

Who is the girl _____ a letter there? A writing B written C to write D being written

解答：

A
writing做伴隨狀語,「那個在寫信的女孩是誰?」
女孩和寫信之間是主動關係,不能用過去分詞,排除BD
做伴隨狀語,用動詞ing形式
Who is the girl writing a letter there = Who is the girl (who is) writing a letter there