<p>Forest fires, long [a. thought to be] a detriment to the environment, [b. are] now understood [c. not only] to be unavoidable but also to be [d. a boon] to the forests. [e. no error]</p>
<p>Why is "a boon" incorrect? I thought forest fires was plural so it should be "but also to be boons to the forests"</p>
<p>The answer is E.</p>