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: two positive integers differ by 4, and sum of their reciprocals is 2. then one of the numbers is
Options: a. a. 3, b. 2, c. c. 5, d. d. 21, e. e. 28
[EX A]: b

[EX Q]: Problem: a and b together can complete work in 6 days. a alone starts working and leaves it after working for 4 days completing only half of the work. in how many days it can be completed if the remaining job is undertaken by b ?
Options: a. 4, b. 12, c. 5, d. 6, e. 7
[EX A]: b

[EX Q]: Problem: the compound interest on $ 30,000 at 7 % p. a is $. 4347. find out the period ( in years ) ?
Options: a. 1, b. 1.5, c. 2, d. 2.5, e. 3
[EX A]:
c