Final answer:
Patents are a government rule that gives the inventor exclusive rights to their invention for a limited time, typically 20 years.
Step-by-step explanation:
A patent is a government rule that gives the inventor the exclusive legal right to make, use, or sell an invention for a limited time.
The correct answer to your question is B. 20 years. Patents are typically granted for a period of 20 years. This allows the inventor to have exclusive rights to their invention during that time and gives them an opportunity to earn a return on their investment.
It's important to note that the 20-year time period for a patent is somewhat arbitrary and is meant to strike a balance between allowing the inventor to profit from their invention and preventing them from holding a monopoly on the market indefinitely.