I like this music.Who's it wirtten ____?A.in B.by C.on

B
这个by 为：被
这样理解
it is wirtten by him 这个句子对him提问就成了
Who's it wirtten by？或者 by Whom is it wirtten

选择B. By

I like this music.Who's it wirtten ____?A.in B.by C.on
选B
还原成原来的语序,就是【the music is written by who】
所以用by

B