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 spherical balls lie on the ground touching. if one of the balls has a radius of 9 cm, and the point of contact is 10 cm above the ground, what is the radius of the other ball ( in centimeters ) ?
Options: a. 50 / 4, b. 70 / 6, c. 80 / 7, d. 90 / 8, e. 100 / 9
d