15. By the time we arrive at the theater, the movie ______ already ended.
(A) had
(B) shall
(C) will have
(D) has been

By the time we arrive at the theater是現在式，所以推測後方的句子是未來即將完成的句型。所以要選the movie will have already ended.

1.   [ By the time ]是連接詞，假如它引導的子句是描述未來，主要子句要用未來完成式。例句如下：

      a) By the time they are released, they will have learned some skills to begin their new lives.

      b) By the time Dad comes home, I will have finished my homework. 

2.   [ By the time ]引導的子句也可以描述過去，主要子句要用過去完成式。例句如下：

      a) By the time I finished my homework, my young brother had gone to bed.

      b) By the time I got to Maggie’s place, she and her friends had all waited for me.

http://twenglishstudy1.pixnet.net/blog/post/299829905-%E8%8B%B1%E6%96%87%E5%A4%A7%E8%80%83%E5%B8%B8%E8%80%83%E9%A1%8C-%E2%80%93-9--%5B-by-the-time-%5D 

未來完成式
主詞 + will have + p.p. (過去分詞)