Final answer:
Dirty air filters can both allow impurities to damage a vehicle's engine (A) and decrease fuel efficiency (B), making option D the correct answer.
Step-by-step explanation:
Dirty air filters in vehicles can lead to a couple of problems that affect the engine and fuel efficiency. Firstly, a dirty air filter can allow impurities to enter the engine, causing damage and wear. Over time, this can result in the need for repairs and can diminish the longevity of the engine. Secondly, when an air filter is clogged, it restricts the airflow to the engine, which can decrease fuel efficiency because the engine has to work harder to draw in air. This means that more fuel is consumed to achieve the same level of performance as compared to when a clean filter allows for optimal air passage.
Appropriate vehicle maintenance, including regularly changing filters, helps protect the engine from damage and maintains the vehicle's fuel efficiency. It is not accurate to say that a dirty air filter can increase fuel efficiency; it can only decrease it or allow engine damage due to impurities.
Therefore, the correct answer to the question is D. both A and B: dirty air filters can allow impurities to damage the vehicle's engine and decrease fuel efficiency.