OPT:
[1]. roads;
[2]. Fuel cells;
[3]. solids;
[4]. fossilized animals;
[5]. power;
[6]. engines;
[7]. energy;
[8]. Electrical;
Q: Fact 1: Using a gasoline engine requires burning gasoline. Fact 2: Oil and gasoline are fossil fuels. Given the two facts above, fossil fuels are burned in many what?
A:
[6].