I know this because A states that the training is not worth the cost, which is correct a little, but it does not state the right way. B is the wrong way around, and C is not even relavent to the text. Therefore you can conclude that D is correct. I JUST WANT THE RIGHT ANSWER TO BE IN THIS!
The least common multiple of 9 and 6 is 18 so in 18 months she will replace the bulb for the 2nd time and the air filter for the 3rd time. It will be the first time they are replaced together.