Question

when should the air filter be checked?
a) at every oil change.
b) if the vehicle is operating poorly.
c) only when the customer request it to be.
d) once a month.

Explanation:

Checking the air - filter at every oil change is a common and recommended practice to ensure the vehicle's engine performance and efficiency. Operating poorly could be due to many factors and not just the air - filter. Relying only on customer request may lead to neglect. Checking once a month may be too frequent for normal driving conditions.

Answer:

A. At every oil change.