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: a dog is tied to a tree by a long nylon cord. if the dog runs from the due north side of the tree to the due south side of the tree with the cord extended to its full length at all items, and the dog ran approximately 30 feet, what was the approximate length of the nylon cord, in feet ?
Options: a. 30, b. 25, c. 15, d. 10, e. 5
d