Most of the world's automobiles run on energy acquired from petroleum. Like all fossil fuels, petroleum is a nonrenewable energy resource. This means that it is a resource that cannot be replaced as quickly as humans use up existing supplies. Humans face a problem because once petroleum reserves begin to run out, there won't be enough fuel to power all the automobiles in the world. Which of the following is the best potential long-term solution to this specific problem of energy supply?