当which是指代宾语时 作为引导词的which可以省略
事实上 只要是先行词充当从句中的宾语 引导词都可以省略
比如
I know the girl (who/that) he is talking with.
He gave me a book (which/that) he had already read many times.
至于你说的 it is 开头 应该是强调句 根本不用which 只用that 或who
It is his daddy who helps me without pay.