<p>This is a <span style="color:#8e44ad;"><strong>Detail Question</strong></span>, since it says "According to the passage". We have to select the answer choices that are stated in the passage.</p>
<p><span style="color:#27ae60;"><span style="font-size:20px;">The Right Answers</span></span></p>
<ul>
<li><span style="color:#27ae60;">A:</span> The passage states that the "unintentional flood in 1983" reduced the population of trout. The "reduction" is a negative effect on the population, so this answer choice is correct.</li>
<li><span style="color:#27ae60;">C:</span> The passage states that the 1983 flood was "unintentional". This means it was not deliberately caused for any purpose, including scientific purposes.</li>
</ul>
<p><span style="color:#e74c3c;"><span style="font-size:20px;">The Wrong Answer</span></span></p>
<p><span style="color:#e74c3c;">B:</span> This choice has no evidence. We only know about the effect the flood had on the NONNATIVE trout, we don't know about its effect on the "native fish population".</p>