They are English cars.(单数）

They are English cars.(改为单数形式) Are they American cars?（改为单数） These are English cars.（改为单数句） Those are English cars.改为单数是什么? They are English cars改为否定句 They are English exercise-books.(改为单数句型） They are English books (5改为单数形式） They are English watches.(改为单数形式） They are English boys.(改为单数形式） they are English books.(改为单数形式) they are an English book变单数 Are they English girls?改为单数.