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: if a and b are the roots of the equation x 2 - 9 x + 9 = 0, then the value of a 2 + b 2 is :
Options: a. 63, b. 24, c. 17, d. 6, e. 5
a

Problem: what is the total number of integers between 10 and 50 that are divisible by 2 ?
Options: a. 18, b. 19, c. 22, d. 25, e. 10
b

Problem: pipe a can fill a tank in 6 minutes and pipe b cam empty it in 24 minutes. if both the pipes are opened together after how many minutes should pipe b be closed, so that the tank is filled in 30 minutes ?
Options: a. 18, b. 27, c. 96, d. 27, e. 21

c