[問題]賴水信高分托福文法p.33(3)

[Lucy Tsai]

如題:賴水信高分托福文法p.33(3)
There will be a magnificent party held in celebration of a thirtieth anniversary of Mr. and Mrs. Brown's marriage.
答案是(C)改為the thirtieth
但是為何那個celebration前面不用加冠詞呢?麻煩知道的人可以回答嗎?
謝謝!