The boy is reading a book which was written by Mark Twain 改为同义句咋改

the book (which is) being read by the boy was written by Mark Twain。。

括号里的单词 加上或者不加都可以

一楼的答案也可以，但是稍显牵强，且与原句侧重点不一样，最好改成：

The boy is reading a book which was written by Mark Twain.

=The boy is reading a book written by Mark Twain.

或直接简化成：The boy is reading a book by Mark Twain.

这句子好晕咧~

The boy is reading a book written by Mark Twain 。