are these your english books?no,( )aren’t .i thank they’re(

are these your english books?no,( )aren't .i thank they're( )books.
are these your english books?
no,( )aren't .i thank they're( )books.
怎么填,并说明理由问题补充：
C.they their D.they them

they
their
选c
再问： 为什么 说明理由 thanks
再答： 第一个空是代词，指代 english books，第二个空是填形容词性物主代词，他们的。