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.

[EX Q]: Problem: the probability of two events a and b are 0.25 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. 0.05, e. 0.6
[EX A]: c

[EX Q]: Problem: if the range q of the 6 numbers 4, 314, 710 and x is 12, what is the difference between the greatest possible value of x and least possible value of x ?
Options: a. 0, b. 2, c. 12, d. 13, e. 15
[EX A]: d

[EX Q]: Problem: a swimmer can swim in still water at 4 km / h. if the speed of the water current is 2 km / h, how many hours will the swimmer take to swim against the current for 5 km ?
Options: a. 2.0, b. 2.5, c. 3.0, d. 3.5, e. 4.0
[EX A]:
b