答案：sewing
介词by 后跟动名词- 动词ing形式
不明白的再问哟,请及时采纳,多谢!还有一题哦、、 the students in 12th grade are called (senior) in the USA.括号里的那个单词要不要用复数形式？本句子时复数，所以名词也用复数 seniors客气