Physics question.

As mentioned in my previous post, I had to miss out on my scholarship class.
Anyway I caught up with one of my friends in the class and he told me that we got a question to work on during the week.

I found this quite hard and I hoped one of you might be able to help.

To understand the question you need to look at a diagram:
http://i26.tinypic.com/11j6hx4.jpg

The diagram shows a mass hanging off a thin string (String A). A thicker string is attached to the bottom of the mass (String B).

The question is:
If String B is pulled very gently, String A breaks/rips and
if String B is pulled very quickly, String B breaks/rips.
Why?

I am thinking this might have to do with elasticity constants and impulse but please answer with some reasonable detail. Thanks.