2道GMAT逻辑,The ancient Nubians inhabited an area in which typh

2道GMAT逻辑,
The ancient Nubians inhabited an area in which typhus occurs,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.Infectious diseases other than typhus to which the ancient Nubians were exposed are unaffected by tetracycline.
B.Tetracycline is not rendered ineffective as an antibiotic by exposure to the processes involved in making bread and beer.
C.Typhus cannot be transmitted by ingesting bread or beer contaminated with the infectious agents of this disease.
D.Bread and beer were the only items in the diet of the ancient Nubians which could have contained tetracycline.
E.Typhus is generally fatal.
Which of the following most logically completes the passage?
Using new detection techniques,researchers have found trace amounts of various medicinal substances in lakes and rivers.Taken in large quantities,these substances could have serious health effects,but they are present in quantities far too low to cause any physiological response in people who drink the water or bathe in it.Nevertheless,medical experts contend that eliminating these trace amounts from the water will have public health benefits,since __________.
A.some of the medicinal substances found in lakes and rivers are harmless to humans even if taken in large quantities
B.some of the medicinal substances found in lakes and rivers can counteract possible harmful effects of other such substances found there
C.people who develop undesirable side effects when being treated with medicines that contain these substances generally have their treatment changed
D.most medicinal substances that reach lakes or rivers rapidly break down into harmless substances
E.disease-causing bacteria exposed to low concentrations of certain medicinal substances can become resistant to them