A scientist used radiation to change the mutation rates of a population of bugs. He then measured how many generations it took for bugs with different mutation rates to adapt to a new background by natural selection. On the graph below, the x-axis represents the mutation rate, and the y-axis represents the number of generations it took for the bugs to reach an average fitness of 90%. Which of the following statements is most accurate?

A. Adaptation occurred most quickly when the mutation rate was lowest.
B. Adaptation occurred more quickly as the mutation rate increased. However, this effect was relatively small for mutation rates above 1%.
C. Adaptation occurred twice as quickly when the mutation rate was 10% as when the mutation rate was 5%.
D. There is no relationship between the mutation rate and how quickly adaptation occurred.