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.

[Q]: Problem: dhoni bought 5 toys with the average cost of $ 10. if david also buys the sixth toy with the price of $ 16, what is the average ( arithmetic mean ) price of those 6 toys ?
Options: a. $ 10, b. $ 7, c. $ 8.5, d. $ 9, e. $ 11
[A]: e


[Q]: Problem: points a, b, c, and d, in that order, lie on a line. if ab = 3 cm, ac = 4 cm, and bd = 8 cm, what is cd, in centimeters ?
Options: a. 7, b. 2, c. 3, d. 4, e. 5
[A]: a


[Q]: Problem: marginal cost is the cost of increasing the quantity produced ( or purchased. by one unit. if the fixed cost for n products is $ 15,000 and the marginal cost is $ 200, and the total cost is $ 25,000, what is the value of n ?
Options: a. 30, b. 50, c. 60, d. 80, e. 100
[A]:
b