According to the law of conservation of energy, energy can't be created or destroyed, just used and converted into different forms. The amount of energy in a system does not change. For example, when a creature dies, the chemical energy in its muscles and blood becomes nutrients that feed the earth. But we are not able to use all of the energy we receive. For example, some energy is wasted as friction between the moving parts of machines; some energy may be converted into thermal energy and radiated into the air, which we can't use. Today, we are faced with the problem of finding the best sources of energy for us to use with the least amount of waste.
We divide our energy resources into two categories: renewable and nonrenewable. Nonrenewable energy resources are those that we can't easily replace. For example, that chemical energy from plants stored as petroleum, natural gas or coal is called fossil fuels. As their name implies, fossil fuels took millions of years to produce; they're nonrenewable because we can't wait millions of years to produce more.
It is important to be careful when using nonrenewable resources because
A
there is a limited supply of them and we can’t make any more.
B
harnessing their energy is a very dangerous process.
C
they are more likely to produce excess thermal energy that we can’t use.
D
there is a possibility of finding dinosaur bones in petroleum or coal deposits.
A
there is a limited supply of them and we can’t make any more.