请问Our classroom is as large as theirs句中的theirs 为什么要用复数形式?

请问Our classroom is as large as theirs句中的theirs 为什么要用复数形式? Our classroom is four times as large as theirs和Our classroom our classroom is as clean as theirs的同义句是什么 our hall is ( )theirs.A twice as large as B twice so large C Our classroom is as big as ___. 是填theirs 还是their Our classroom is ( ) ( )than theirs. Our classroom is the same size as theirs=( )没有限格子 our school life is as __ as theirs(colour) Our classroom is __________larger than theirs. Our classroom is （ ）cleaner than theirs Our garden isn't so ( )as theirs.A.larger B.large C.bigger D Our classroom is on the second floor.Theirs is d____.