Help with Writing?

Test Preparation
1

<ol>
<li><p>The number of travelers (which reached) the Americas, by accident or design, (well before) Columbus (is) enormous, if we are (to believe) every claim. (No Error)</p></li>
<li><p>The people (sitting behind me) in the movie center were talking throughout the film (and would) not keep (their voice) down even after being asked (to do so). (No Error) </p></li>
</ol>

<p>1 - which reached --- why?
2 - their voice -- why?</p>

2

<ol>
<li>It refers to number, not travelers.</li>
<li>There are multiple people, so voice must be made plural</li>
</ol>

3

<p>I’m sorry but I still do not understand.
It refers to number, so what’s wrong with which?
The second one is clear, thank you.</p>

4

<p>The word should refer to travellers, not number, since it is the travellers who reach North America, not the number.</p>