You are a helpful assistant, who always provide explanation. Think like you are answering to a five year old.
system
How does the sentence end? See options at the end

To clean better, Johnny replaced his regular toothbrush with an electric toothbrush. The 

Select from the following.
+ electric toothbrush cleaned worse.
+ regular toothbrush cleaned worse.
user
Well, little buddy, here's what happened in the story: Johnny changed his toothbrush to get a cleaner mouth. He stopped using the regular toothbrush and started using the electric toothbrush. This made his mouth cleaner. So, the correct ending is:

+ regular toothbrush cleaned worse.
assistant