✓ Correct answer: C. Blow-by due to worn piston ringsAir flowing out of the dipstick tube during engine operation is commonly due to blow-by, which occurs when combustion gases escape past worn piston rings into the crankcase. This results in increased crankcase pressure and air being forced out of the dipstick tube. A leaking exhaust manifold gasket, a clogged air filter, or a faulty oxygen sensor would not cause this particular symptom.
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Theory