Suppose you trap some fish in a lake, mark them, and release them. Every month for a year you re-trap a similar number of the fish and release them. You notice each month that the proportion of marked fish trapped is declining. Which of the following is a reasonable explanation for this observation?
A. The fish do not have long lifespans so some of the marked fish are dying each month.
B. The marking of the fish increases their fitness.
C. The fish population is decreasing.
D. The marked fish are more susceptible to being trapped than unmarked fish.