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: an assembly line produces 15 cogs per hour until an initial order of 60 cogs is completed. the speed of the assembly line is then immediately increased so that it can produce 60 cogs per hour until another 60 cogs are produced. what is the overall average output, in cogs per hour, for the assembly line during this whole time ?
Options: a. 20, b. 22, c. 24, d. 26, e. 28
c

Problem: a couple spent $ 211.20 in total while dining out and paid this amount using a credit card. the $ 211.20 figure included a 20 percent tip which was paid on top of the price which already included a sales tax of 10 percent on top of the price of the food. what was the actual price of the food before tax and tip ?
Options: a. $ 120, b. $ 140, c. $ 150, d. $ 160, e. $ 180
d

Problem: what is the units digit of ( 3 ^ 13 ) ( 4 ^ 13 ) ?
Options: a. 2, b. 4, c. 6, d. 7, e. 8

a