题干
— How old is Mary?
— She is having her______ birthday this weekend.
A：four
B：the four
C：fourth
D：the fourth
答案(点此获取答案解析）
C
— How old is Mary?
— She is having her______ birthday this weekend.
A：four
B：the four
C：fourth
D：the fourth
②a的结构通式可表示为