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.

[Q]: Problem: a walks at 10 kmph and 4 hours after his start, b cycles after him at 20 kmph. how far from the start does b catch up with a ?
Options: a. 100 km, b. 150 km, c. 80 km, d. 120 km, e. 200 km
[A]: c

[Q]: Problem: if the sides of a cube are in the ratio 5: 3. what is the ratio of their diagonals ?
Options: ['a. 5: 9', 'b. 9: 3', 'c. 5: 4', 'd. 3: 5', 'e. 5: 3']
[A]: e

[Q]: Problem: of the total amount that jill spent on a shopping trip, excluding taxes, she spent 60 percent on clothing, 20 percent on food, and 20 percent on other items. if jill paid a 10 percent tax on the clothing, no tax on the food, and an 10 percent tax on all other items, then the total tax that she paid was what percent of the total amount that she spent, excluding taxes ?
Options: a. 10, b. 20, c. 30, d. 25, e. 15
[A]:
a