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: if the perimeter of a rectangular garden is 1800 m, its length when its breadth is 400 m is ?
Options: a. 500, b. 600, c. 700, d. 800, e. 900
a