Q: 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 certain airline ' s fleet consisted of 90 type a planes at the beginning of 1980. at the end of each year, starting with 1980, the airline retired 3 of the type a planes and acquired 4 new type b plans. how many years did it take before the number of type a planes left in the airline ' s fleet was less than 50 percent of the fleet ?
Options: a. 11, b. 12, c. 13, d. 14, e. 15
A:
c