Definition: You are given a math word problem and you are supposed to make use of mathematical concepts like probability, combinatorics, linear equations, sequences/series, or simple operators like addition, subtraction, division, or multiplication to answer the given question. Choose the best option from the 5 given options and report that option.
Input: Question: The average age of a class of 20 students is 21 years. The average increased by 1 when the teacher's age also included. What is the age of the teacher?

 Option A: 40

 Option B: 41

 Option C: 42

 Option D: 45

 Option E: 43

Option C