怎么把Are those your keys改为单数形式?

怎么把Are those your keys改为单数形式? 1-Are those your sons?(改为单数形式） 2-Are those his keys?(改为单数形式） Those are your books.改为单数形式要怎么改? Those are some keys.(改为单数） these are your keys 改为单数句 Those are our teachers怎么改为单数形式急! Those are rulers 改为单数形式 快 are those japanese cars?改为单数形式 Those are erasers 改为单数形式 Those are her photos改为单数形式~ Those are oranges 改为单数形式 Those are gloves改为单数形式