[gnus] Fetch only new news from last visit

[Diep Pham]

I'm trying to use gnus to read news from gmane.org. But it is quite
confusing to use.

I subscribed to a group, when I try to access the group, gnus ask me:
"How many articles do you want to ...". I don't think fetch all news is
good idea so I choose 100. I read all news and close gnus. When I open
gnus and try to access the newsgroup again, gnus still ask me "How many
articles do you want to ...". I enter 100 again and endup fetching older
news than the last time I visit the group. So my question:

* is there a way that can prevent fetching older than last visit?
* is there a way to disable the prompt "How many ariticles ..." and just
  fetch all new news like a normal news reader?