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: a can do a piece of work in 4 hours; b and c together can do it in 3 hours, while a and c together can do it 2 hours. how long will b alone take to do it ?
Options: a. 8 hours, b. 10 hours, c. 12 hours, d. 24 hours, e. none of these
c

Problem: the difference between the compound interest compounded annually and simple interest for 2 years at 20 % per annum is rs. 216. find the principal ?
Options: a. 2277, b. 2667, c. 5400, d. 2766, e. 1811
c

Problem: a solution contains 8 parts of water for every 7 parts of lemonade syrup. how many parts of the solution should be removed and replaced with water so that the solution will now contain 25 % lemonade syrup ?
Options: a. 1.5, b. 1.75, c. 2.14, d. 2.34, e. 2.64

c