today is tony is birthday lt is his --- birthdaya\11b\ 11stc\ 11th说理由!

today is tony is birthday lt is his --- birthday
a\11
b\ 11st
c\ 11th
说理由!

选C
第十一个生日,A是错的,所以在B和C中挑,B的写法不对,1st,21st.但第十一个就是11th.所以选C

选C
要选择一个序数词来修饰birthday
11th即eleventh意为第十一个~