The air __ has been much better since the factories closed. natural quality gasoline

## Step 1<br />The sentence is about the improvement of the air since the factories closed. The word that needs to be inserted should be related to the air and its condition.<br />## Step 2<br />Looking at the options, "natural" and "gasoline" are not suitable in this context. "Natural" is an adjective that describes something as being natural, and "gasoline" is a type of fuel. Neither of these words fits the context of the sentence.<br />## Step 3<br />The word "quality" is the most suitable choice. It refers to the standard or level of something. In this context, it refers to the standard or level of the air.<br />## Step 4<br />Therefore, the correct sentence is "The air quality has been much better since the factories closed." This means that the level or standard of the air has improved since the factories closed.