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.

[EX Q]: Question: Which crust lies deep inside the ocean? Answer: oldest
[EX A]: Oldest crust lies deep inside the ocean.

[EX Q]: Question: What is secreted before ejaculation to neutralize acidic urine remaining in the urethra? Answer: clear mucus
[EX A]: Clear mucus is secreted before ejaculation to neutralize acidic urine remaining in the urethra.

[EX Q]: Question: What describes how much matter is in a certain amount of space? Answer: density
[EX A]:
Density describes how much matter is in a certain amount of space.