9. This question is based on the following paragraph. Both petroleum and coal are made up of complex carbon-based molecules, and both originated with living creatures of some kind. Both are vital sources of energy for the modern world and both were formed by geologic processes over millions of years. However, petroleum was mainly formed from the remains of ocean-dwelling microorganisms. Coal, on the other hand, originated from decayed vegetation in ancient swamps and bogs. In any case, it took millions of years for both coal and oil to be produced. This is the case because it took that much time for overlying sediments to produce the unimaginable heat and pressure that would one day allow us to harvest these energy resources. What is one clue word or phrase in the passage that indicates a contrast? A. Both B. In any case C. However D. Because

A. is not the answer since 'both' does not contrast, but instead shows similarity

b. is not the answer either. it does not indicate contrast (or anything, really) and the second part of that sentence shows similarities between coal and petroleum.

c. however is usually used to introduce a statement that contrasts. This happens in this paragraph, too. The first part of the statement tells how petroleum is made from dead microorganisms. The second part of the statement is introduced with "on the other hand," a common phrase used to compare and contrast two things or ideas. The sentence then states that coal was made from dead vegetation. These are two characteristics that are different about coal and petroleum that are portrayed in a contrasting manner. Therefore c. however is the correct answer.

D. is not the answer. "because" is simply used to explain an idea.