GMAT Critical Reasoning (CR) | Re: Some batches of polio vaccine used around 1960 were

Hi Experts,
Can you help me to distinguish if this is an assumption question or strengthen question? Furthermore, what does phrase - is not undercut - suggest?
I am presenting my analysis:
Some batches of polio vaccine used around 1960 were contaminated with SV40, a
virus that in monkeys causes various cancers.

This is a fact.

Some researchers now claim that
this contamination caused some cases of a certain cancer in humans,
mesothelioma.

This sentence is claim by researchers so it can be
...

---