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: the difference between the heights of two trees is 20 cm. 10 years ago, the taller one was thrice as taller as the shorter one. the present height of the taller tree is
Options: a. 300 cm, b. 15 cm, c. 20 cm, d. 10 cm, e. 40 cm
a