Answer: A. The dark bacteria showed resistance to the drug.
Step-by-step explanation:
According to the given situation, it is evident from the bacterial culture in the first plate containing the generation 1 bacteria. In generation 1 the bacteria which were exposed to the drug of the type which were resistant as well as non-resistant to the drug. But after successive generations the less resistant bacteria became susceptible to the drug and decline in numbers. Upto generation 20 only the resistant bacteria survived. Those were the dark bacteria.