<p>1: the first part of the second clause is “I like to” not “I like”</p>
<p>2: subject is “air pollution”</p>
<p>3: “in contrast with” and there is nothing wrong with original sentence</p>
<p>4: not sure, but you don’t need one, and you want to keep it concise when possible</p>
<p>5: there is no verb in the original sentence, </p>
<p>6: technically nothing wrong, but if there was an answer choice that read “the United States and Canada set limits” I would be inclined pick that</p>