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: john and david can finish a job together in 2 hours. if john can do the job by himself in 3 hours, what percent of the job does david do ?
Options: a. 36.33 %, b. 33.33 %, c. 39.33 %, d. 40.33 %, e. 33 %
b

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

Problem: a retailer bought a machine at a wholesale price of $ 108 and later on sold it after a 10 % discount of the retail price. if the retailer made a profit equivalent to 20 % of the whole price, what is the retail price of the machine ?
Options: a. 81, b. 100, c. 120, d. 135, e. 144

e