<p>During the labor dispute, barrels of potatoes were emptied across the (highway, and they thereby blocked it to all the traffic)</p>
<p>The answer is: (highway, thereby blocking all traffic) Why is this correct? Don't you need a conjunction to join them?</p>
<p>(Along) the curve of islands as the Florida Keys (lies) a reef of living coral, (the only one) of (a kind) in the continental United States. </p>
<p>The answer is D. Why? I do not see anything wrong with it.</p>
<p>First question: “thereby” is an adverb, introducing the effect of the barrels of potatoes, so it does not require “and.” The usage “thereby xxx-ing” is fairly common in formal written English. You can look up “thereby” in Wiktionary to see more examples.</p>
<p>Second question: The sentence should say that the coral reef is the only one of “its kind.” There are many things/people in the continental US that are “one of a kind.” Therefore, the reef cannot be the “only one of a kind.” But it is the only reef of living coral in the US, so it is the only one of its [own] kind.</p>