>df</code> is a survey dataset.</p>
<ul>
<li><p><code>a, b, c, d</code> = Survey questions</p></li>
<li><p><code>-99</code>: non-responses</p></li>
<li><p>Your goal: replace <code>-99</code> with <code>NA</code></p></li>
</ul></li>
</ul>
<div class="sourceCode" id="cb23"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb23-1"><a href="Intro.html#cb23-1"></a><span class="co"># Data</span></span>
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