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: for the past n days, the average ( arithmetic mean ) daily production at a company was 50 units. if today ' s production of 110 units raises the average to 55 units per day, what is the value of n ?
Options: a. 9, b. 11, c. 13, d. 15, e. 17
[A]: b

[Q]: Problem: two trains of equal are running on parallel lines in the same direction at 45 km / hr and 36 km / hr. the faster train passes the slower train in 36 sec. the length of each train is ?
Options: a. 45, b. 88, c. 66, d. 55, e. 22
[A]: a

[Q]: Problem: a man whose speed is 4.5 kmph in still water rows to a certain upstream point and back to the starting point in a river which flows at 0.5 kmph, find his average speed for the total journey ?
Options: a. 4, b. 3.5, c. 3, d. 3.1, e. 2.8
[A]:
d