TASK DEFINITION: In this task, you are given a question and an answer, you would be asked to create the sentence based on the Question-Answer provided. It should be contained within the Question-Answer provided.
PROBLEM: Question: Over 90% of the energy we use comes originally from what? Answer: sun

SOLUTION: Over 90% of the energy we use comes originally from the sun.

PROBLEM: Question: What is the key to the demographic transition? Answer: reduced family size

SOLUTION: Reduced family size is the key to the demographic transition.

PROBLEM: Question: The frequency of a wave is the reciprocal of what other attribute of the wave? Answer: period

SOLUTION:
The frequency of a wave is the reciprocal of the period of the wave.