Sentence A: It is a common practice to give two antibiotics simultaneously to treat some diseases. Sentence B: The chance of two mutations in the same cell is the product of the single mutation for each.

Sentence A is correct and sentence B is the right explanation of sentence A. It is a common practice to give two antibiotics simultaneously to treat some diseases because if the mutation rate to first antibiotic is supposed to be 10^-6 and mutation rate to the second antibiotic is 10^-8 per cell division, then the probability that both the mutations would occur in the same cell is 10^-6 x 10^-8 = 10^-14 This means that the chance of two mutations in the same cell is the product of the single mutation for each.