rain forests contain more trees, and in higher density, than any other ecological system.
a. than,
b. than that of,
c. than those of,
d. than does,
答案是d，有点不理解。我觉得比较对象是rain forests 和any other ecological system，即使选d也应该是than any other ecological system does.而且到底什么时候助动词在比较中是可以省略的？什么时候不可以呢？
谢谢啊 | 
