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: a cyclist bikes x distance at 8 miles per hour and returns over the same path at 10 miles per hour. what is the cyclist ' s average rate for the round trip in miles per hour ?
Options: a. 8.1, b. 8.9, c. 8.6, d. 8.3, e. 9.0
b

Problem: a bag contains 10 green and 8 white balls. if two balls are drawn simultaneously, the probability that both are of the same colour is
Options: a. 8 / 15, b. 21 / 44, c. 3 / 5, d. 11 / 15, e. 7 / 15
b

Problem: a rectangular field has area equal to 150 sq m and perimeter 50 m. its length and breadth must be ?
Options: ['a. 15 m, 15 m', 'b. 15 m, 90 m', 'c. 16 m, 10 m', 'd. 15 m, 10 m', 'e. 15 m, 30 m']

d