The Following Question Refers to the Figure

Multiple Choice

The following question refers to the figure.
In this eight-year experiment, 12 populations of E. coli, each begun from a single cell, were grown in low-glucose conditions for 20,000 generations. Each culture was introduced to fresh growth medium every 24 hours. Occasionally, samples were removed from the populations, and their fitness in low-glucose conditions was tested against that of members sampled from the ancestral (common ancestor) E. coli population.

If the vertical axis of the figure above refers to relative fitness, then which of the following is the most valid and accurate measure of fitness?

A) number of daughter cells produced per mother cell per generation
B) average swimming speed of cells through the growth medium
C) amount of glucose synthesized per unit time
D) number of generations per unit time