<p>Page 150.</p>
<li>By virtue of [its] size and supersensitive electronies, modern radio telescopers are able to gather mopre waves and discriminate among them with greater precision than earlier versions could.</li>
</ol>
<p>The answer is its.
Why?
Is it because its is ambiguous?</p>
<li>The starling is such a pest in rural areas that it has become necessary to find ways of controlling the growth of [their] population.
The answer is their.
Is it because the starling is singular and their is a plural noun?</li>
</ol>