A Fleet of Rental Cars - All the Same Make $\mu$

A fleet of rental cars - all the same make, model, and year - has a mean fuel efficiency of 25.6 miles per gallon (mpg) . A random sample of 54 cars are selected and the air filter of each is replaced with a new one. Let $\mu$ be the population mean fuel efficiency score that would occur if every car's air filter were replaced. The air filter change is deemed effective if $\mu > 25.6 \mathrm { mpg }$
A test is made of $H _ { 0 } : \mu = 25.6 \text { versus } H _ { 1 } : \mu > 25.6$
Assume that the air filter changes are effective but the conclusion is reached that the chan might not be effective. Which type of error, of any, has occurred?

A) No error - correct decision
B) Type I
C) Mechanical failure
D) Type II