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: 18.05 ã — 0.001 = ?
Options: a. 0.1805, b. 0.01805, c. 1.805, d. 0.001805, e. none of these
b

Problem: two pipes a and b can fill a tank in 36 hours and 46 hours respectively. if both the pipes are opened simultaneously, how much time will be taken to fill the tank ?
Options: a. 20.195 hrs, b. 21.195 hrs, c. 22.195 hrs, d. 23.195 hrs, e. 20.495 hrs
a

Problem: the sum of the squares of three numbers is 138, while the sum of their products take two at a time is 131. find the sum ?
Options: a. 10, b. 30, c. 20, d. 50, e. 40

c