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

## Step 1<br />The problem is a fill-in-the-blank type, where we need to choose the most appropriate word from the given options to complete the sentence. The sentence is "The air __ has been much better since the factories closed."<br /><br />## Step 2<br />We need to consider the context of the sentence. The sentence talks about the improvement of the air since the factories closed. This suggests that the word we need to fill in the blank should be related to the air quality.<br /><br />## Step 3<br />Looking at the options, "quality" is the most suitable word. The word "quality" fits the context of the sentence and makes the sentence meaningful.<br /><br />## Step 4<br />The other options "gasoline" and "natural" do not fit the context of the sentence. "Gasoline" is a type of fuel and "natural" is an adjective that doesn't make sense in this context.