GMAT Critical Reasoning (CR) | Re: Exposure to certain chemicals commonly used in elementary

Negate A.
The number of school nurses employed by Renston’s elementary schools has decreased over the past ten years.
Now assume that Total no of student remain same over ten years.
No. of student sent to one nurse will increase for sure because there is a decline in nurses available hence the proportion increases. And conclusion will fall apart, which state only two reasons.

But if collectively all the nurses report this, then there is no effect. I am not sure, pl somebody confirm.