Answer:
a. unnatural occurrences
Explanation: although assignment 6 covers what affects the balance of nature extensively. The answer is actually provided in assignment 12, first paragraphed (if printed off) under Human Interaction in the sentence. "Unnatural occurrences or changes to an ecosystem tend to have the most significant impact, to the point that some may not be restored." T
hen introduces applied ecology which is the answer for Q. 19, so review assignment 12 for that answer as well in paragraph 1.
However, the test questions are often given out of order and then re-shuffled after grading so printing off the test ahead of time gives you the graded order and as you take the test you must jump around the paper test first. Hope this helps.