Bell and collaborators painstakingly documented a population of fossil sticklebacks from an ancient freshwater lake over a 20,000-year period. The prevalence of sticklebacks with full and reduced pelvises changed over time. Which is true?
a. In this population, having pelvic spines seemed to provide a selective advantage to stickleback fish.
b. The population of fish with pelvic spines that arrived in the lake at time B evolved a reduced pelvis over time (beginning at time C).
c. By the end of the studied period (time D) there was an approximately equal number of sticklebacks with and without spines. Initially (time A), fish with full pelvises dominated the lake population. d. The evolution of pelvic reduction seen in the fossil record is different from the phenomenon occurring in the lakes in Alaska today.

The fossil record showed that before the time period B, the ancient sticklebacks might have had a huge pelvis which was of no use to them. As a result, through evolution at time B the pelvis reduced in the sticklebacks through natural selection. Having a reduced pelvis must have been advantageous for this ancient stickleback population, just as it is for many current freshwater stickleback populations. Hence, the correct option is B.