<p>^actually, in no.2 “Air pollution” is the subject for the first clause. In the second clause, “only recently [has] the harmful effects of noise pollution [become] known,” “has” disagrees in number with the plural subject “harmful effects of noise pollution.” So, “have” should have been used instead of “has.” It is much easier to find errors in complex sentences/clauses if you rearrange the words in your mind to make them simpler. For instance, if you had turned the latter clause into “the harmful effects of noise pollution [has] [become] known only recently,” you probably would have found it much easier to note the subject-verb disagreement. </p>
<p>Also, for no. 5, “is” is a verb. I wish I could provide a normative explanation for the problem with the original sentence but I’m afraid don’t know enough grammar rules to do so.</p>