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: 10, 1000, 100000, 10000000....
Options: a. 100, 000,000, b. 1,000, 000,000, c. 1, 000,000, 000,000, d. 100, e. 1, 000,000
b

Problem: a man swims downstream 36 km and upstream 18 km taking 3 hours each time, what is the speed of the man in still water ?
Options: a. 7, b. 8, c. 9, d. 2, e. 4
c

Problem: the probability of two events a and b are 0.20 and 0.40 respectively. the probability that both a and b occur is 0.15. the probability that neither a nor b occur is _________
Options: a. 0.45, b. 0.4, c. 0.5, d. 0.55, e. 0.6

d