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.

[EX Q]: Problem: the mean of 50 observations is 200. but later he found that there is decrements of 34 from each observations. what is the the updated mean is ?
Options: a. 165, b. 166, c. 190, d. 198, e. 199
[EX A]: b

[EX Q]: Problem: each of the three people individually can complete a certain job in 3, 4, and 6 hours, respectively. what is the lowest fraction of the job that can be done in 1 hour by 2 of the people working together at their respective rates ?
Options: a. 1 / 3, b. 9 / 20, c. 8 / 15, d. 5 / 12, e. 2 / 3
[EX A]: d

[EX Q]: Problem: the ratio of the radius of two circles is 1: 5, and then the ratio of their areas is ?
Options: a. 1: 7, b. 2: 9, c. 1: 25, d. 3: 7, e. 3: 4
[EX A]:
c