You want to estimate the number of canvas backs (a type of duck) at the Sacramento Wildlife Refuge. In two days you capture 200 canvas backs, mark all of them and release them. One month later you capture another 200 canvas backs, of which 100 were marked by you last month. Calculate an estimate for the population size of canvas backs.
A. 400
B. 100
C. 200
D. 300
E. 600