A location gets its drinking water from wells drilled into the Earth. The people in the location have reported that their drinking water has a foul smell since a few days. A student investigates the cause of the foul smell in the drinking water and concludes that a gasoline spill about a year ago has caused the contamination in groundwater. Why is the student's conclusion most likely invalid?

a. it is based on information obtained from a secondary source
b. there is no published information about gasoline spills in the locality
c. it is biased against gas companies which may not always cause gasoline spills
d. there is a large time gap between the gasoline spill and contamination of water

From the question, we can infer that the location has been getting its supply of water for some time before reporting the change in smell of the water.
Therefore, if the gas spill was the reason for this contamination, the change would have in a sooner time and not take a complete year (or more) to affect the water.

Based on the above explanation, the best answer would be:
D. there is a large time gap between the gasoline spill and contamination of water