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.
--------
Question: Problem: a solution of 70 litres contains oil and water in the ratio 9: x. if 6 litres of water is added to the solution, the ratio becomes 6: 4, find the value of x ?
Options: a. 6, b. 8.5, c. 9, d. 6.5, e. 5

Answer: e


Question: Problem: if a train, travelling at a speed of 90 kmph, crosses a pole in 4 sec, then the length of train is ?
Options: a. 281, b. 125, c. 288, d. 100, e. 121

Answer: d


Question: Problem: the average salary of all the workers in a workshop is $ 8000. the average salary of 7 technicians is $ 12000 and the average salary of the rest is $ 6000. the total number of workers in the shop is ?
Options: a. 15, b. 18, c. 21, d. 25, e. 31

Answer:
c