The ancient Nubians inhabited an area in which typhus occurred, yet surprisingly few of their skeletons show the usual evidence of this disease. The skeletons do show deposits of tetracycline, an antibiotic produced by a bacterium common in Nubian soil. This bacterium can flourish on the dried grain used for making two staples of the Nubian diet, beer and bread. Thus, tetracycline in their food probably explains the low incidence of typhus among ancient Nubians.
Which of the following is an assumption on which the argument relies?
A The tetracycline deposits did not form after the bodies were buried.
B The diseases other than typhus to which the ancient Nubians were exposed would not be affected by tetracycline.
C Typhus is generally fatal.
D Nubian grain became contaminated with tetracycline-producing bacteria prior to being harvested.
E Bread and beer were the only foods eaten by the ancient Nubians which could have contained tetracycline.
A？WHY？ | 
