把下面的句子变成复数形式
问题描述：
把下面的句子变成复数形式
(1)This is a book.
(2)That is a yellow pencil.
(3)He is a good boy.
(4)She is playing on the computer.
答
these are books.
those are yellow pencils
they are good boys
they are playing on the computer(s).