Given the task definition and input, reply with output. Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

Problem: in a park there are two ponds with both brown ducks and green ducks. in the smaller pond there are 45 ducks and in the larger pond there are 55 ducks. if 20 % of the ducks in the smaller pond are green and 40 % of the ducks in the larger pond are green, then what percentage of ducks are green ?
Options: a. 25 %, b. 27 %, c. 29 %, d. 31 %, e. 33 %
d