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: how many positive even integers less than 100 contain digits 5 or 7 ?
Options: a. 10, b. 17, c. 18, d. 19, e. 20
[EX A]: a

[EX Q]: Problem: how many positive integer solutions does the equation 8 x + 2 y = 100 have ?
Options: a. 50, b. 84, c. 16, d. 35, e. 14
[EX A]: b

[EX Q]: Problem: the average of 9 observations was 8, that of the 1 st of 5 being 10 and that of the last 5 being 8. what was the 5 th observation ?
Options: a. 18, b. 12, c. 15, d. 17, e. 19
[EX A]:
a