Question about strep throat

A student with Strep throat (caused by a strain of bacterium) is given a prescription for an antibiotic and is told to take the drug for two weeks. After a week, the student feels better and stops taking the drug. Two months later, the student again shows symptoms of Strep throat and decides to finish the leftover antibiotic rather than going to the doctor again. Two months later, the student develops Strep throat for a third time and returns to the doctor. This time, the antibiotic does not work. The doctor runs a test and discovers that the bacterial strain the student is carrying is antibiotic resistant. What most likely happened?
A.Because the use of antibiotics lowers the effectiveness of the immune system, the student continually reinfected himself with the bacteria. The third time, the student happened to be reinfected with a resistant strain.
B.When the student stopped taking the drug, a small number of bacteria-those that were more drug resistant-still survived in his body. Those bacteria repopulated his throat and over time, drug-resistant alleles became more common.
C.The student must have eaten produce that had been genetically engineered with antibiotic-resistant genes. When he consumed them, the bacteria in the student’s throat picked up these genes through horizontal gene transfer.
D.The antibiotic caused mutations in the bacterium. The more exposure to the antibiotic, the more mutations. Even one random mutation can confer antibiotic resistance.

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