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: 108 ã — 108 + 92 ã — 92 = ?
Options: a. 20120, b. 11500, c. 20128, d. 20660, e. 20228
[EX A]: c

[EX Q]: Problem: the present worth of rs. 2310 due years hence, the rate of interest being 15 % per annum, is :
Options: a. rs. 1440, b. rs. 1750, c. rs. 1680, d. rs. 1840, e. none of these
[EX A]: c

[EX Q]: Problem: solve the equation 2 – x = 2 b – 2 ax, where a and b are real parameters. find for which values of a the equation has for solution a natural number, if b = 7
Options: a. 0, b. 1, c. 2, d. 3, e. 4
[EX A]:
b