Question

the risk to researchers
why is there no health risk for researchers working at the burial site?
plague is now a treatable disease
plague bacteria cannot survive very long in soil
human beings can no longer be affected by the bacteria that cause plague
there are no rats buried nearby and no fleas to carry the disease

Explanation:

Plague bacteria (Yersinia pestis) have specific survival requirements. They need a suitable host - vector (rats - fleas) system for transmission. In a burial site without rats (hosts) and fleas (vectors), the bacteria cannot establish the necessary transmission cycle. While plague is treatable (but this doesn't directly address the risk at the burial site), and humans can still be affected by the bacteria (contradicts option C), and the key factor is the absence of the host - vector system. Plague bacteria can survive in soil for some time, but without the host - vector, the risk to researchers is low.

Answer:

there are no rats buried nearby and no fleas to carry the disease