Here's a problem to solve: A student wants to create a model showing the most important reason why old gasoline should never be poured into the soil. Which of the following must be included in the model?

Among the 4 following options, which is the correct answer?

- A: gasoline evaporating from soil
 
- B: gasoline catching fire in the soil
 
- C: gasoline polluting an old septic system
 
- D: gasoline trickling down into groundwater
 