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.
Q: Problem: the area of a square garden is q square feet and the perimeter is p feet. if q = p + 21, what is the perimeter of the garden in feet ?
Options: ['a. 24', 'b. 28', 'c. 32', 'd. 36', 'e. 40']
A:
b