Fact 1: Heat and pressure change the remains of prehistoric living things into natural gas. Fact 2: Natural gas is the cleanest of the fossil fuels. Given the two facts above, where do fossil fuels come from?

Available choices: (i) Dead prehistoric beings. (ii) Under water. (iii) basic building blocks of life. (iv) dead organisms. (v) decaying trees. (vi) burning coal. (vii) A factory. (viii) Outerspace.
Answer:
(i)