书桌上有堆书there are a pile of books on the desk为什么不能写成there are piles of books on the desk

书桌上有堆书there are a pile of books on the desk
为什么不能写成there are piles of books on the desk

书桌上有堆书there is a pile of books on the desk.( pile 是单数,故用is)　
　而there are piles of books on the desk（不止一堆书）