"Well, I was thinking we might run cross country this fall. For every lock..."
"There is a key!" the three of them said together, bursting into laughter.

What is the meaning of the phrase “For every lock, there is a key” in paragraphs 102-103?
A. Everyone needs to find their answers to their frustrations.
B. Everyone shares the same problems and frustrations.
C. There will always be problems to solve.
D. There is a solution to every problem.