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: two trains of length 100 meters and 200 meters are 450 meters apart. they are moving towards each other on parallel tracks, at speeds of 90 km / h and 72 km / h. after how many seconds will the trains meet ?
Options: a. 6, b. 8, c. 10, d. 12, e. 14
c

Problem: what is the rate percent when the simple interest on rs. 720 amount to rs. 180 in 4 years ?
Options: a. 6.25, b. 7.25, c. 8.25, d. 9.25, e. 10.25
a

Problem: the dimensions of a rectangular solid are 4 inches, 5 inches, and 12 inches. if a cube, a side of which is equal to one of the dimensions of the rectangular solid, is placed entirely within thespherejust large enough to hold the cube, what the ratio of the volume of the cube to the volume within thespherethat is not occupied by the cube ?
Options: a. 10: 17, b. 2: 5, c. 5: 16, d. 25: 7, e. 10: 21
e