The exercise I am doing is as follows:

The following are my questions:
Why the exercise says that Samuel Clarke's argument "allow for the possibility of causal chains with no beginning"?
I reconstruct Samuel Clarke's argument. Is there something wrong or missing?
(1)
[Premise]Every object has an explanation for its existence.(2)
[Premise]All natural objects are dependent, and these objects can only exist if they are caused to exist by something else.(3)
[Definition]Nature is a totality which includes every natural object.(4)
[From (3)]Nature is an object.(5)
[From (1)]There is an explanation for nature's existence.(6)
[From (3), (4)]Nature cannot be the explanation for the existence of itself.(7)
[From (4), (5), (6)]There must be something outside nature. And this thing is the explanation for the existence of nature.(8)
[From (2), (3)]This thing must be independent, otherwise, this thing will be a member of nature.(9)
[From (8)]Something that is self-existent and outside nature must exist.
How Hume objects Clarke's argument? I think that what Hume wants to argue is that
it is nonsense to say a totality has a cause. I wonder whether I am correct.How to evaluate Hume's objection? I have no idea about this question.
Thank you very much!