Teacher:You are given a passage. You need to construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) its answer is the whole paragraph. Avoid creating questions that can be answered correctly without actually understanding the paragraph.
Teacher: Now, understand the problem? Solve this instance: Right.  Fuel additives are helpful for reduction of emissions, although they are not always economical.  For the most part, the US congress has subsidized the creation of ethanol, which tends to be a net cost unless created from waste agricultural products.\nAnother substance that has the same anti-emissions properties as ethanol is MTBE.  There has been a lot of press about MTBE recently due to its tendency to pollute groundwater.
Student:
Why do gas stations mix ethanol into gas?