There are some tractors _____(collect)the rubbish.

There are some tractors _____(collect)the rubbish.
There are some tractors _____(collect)the rubbish.

There are some tractors _____(collect)the rubbish.
collecting 动名词
因为这句主动词是 are

collecting 与tractors构成主动关系 呵
也是现在进行时的用法 there be sb doing sth

collecting
用到are就是现在进行时