求英语高手给详解啊···

问题描述:

求英语高手给详解啊···
Creating a better natural environment in China is a great idea,but ( )the government has difficulty putting into practice in a short period of time.
为什么空格处只能填one而不是that呢?
that不也能用来代a great idea吗,that不是万能么?

one和that从表面上来看都是代词,两个都可同时指物.但这里one可以作泛指,也可有前置的修饰语句.但that却不可以.*提出的好想法其实是很广泛的.