Researchers studying the spread of the Black Plague in 16th-century England claim that certain people survived the epidemic because they carried a genetic mutation, known as Delta-32, that is known to prevent the bacteria that causes the plague from overtaking the immune system. To support this hypothesis, the researchers tested the direct descendants of the residents of an English town where an unusually large proportion of people survived the plague. More than half of these descendants tested positive for the mutation Delta-32, a figure nearly three times higher than that found in other locations.

The researchers’ hypothesis is based on which of the following assumptions?

(A) Delta-32 does not prevent a carrier from contracting any disease other than the plague.
(B) The plague is not similar to other diseases caused by bacteria.
(C) Delta-32 did not exist in its current form until the 16th century.
(D) No one who tested positive for Delta-32 has ever contracted a disease caused by bacteria.
(E) The plague does not cause genetic mutations such as Delta-32.