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 average age of 40 students in a class is 10 years. if teacher ' s age is also included then average increases 1 year then find the teacher ' s age ?
Options: a. 59, b. 55, c. 61, d. 51, e. 36
A:
d