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 cone of height 9 cm with diameter of its base 18 cm is carved out from a wooden solid sphere of radius 9 cm. the percentage of the wood wasted is :
Options: ['a. 45 %', 'b. 56 %', 'c. 67 %', 'd. 75 %', 'e. none of these']
d