<p>Why isn’t the answer “D” (theirs) ?</p>
<p>''Whether the Sumerians were the first people to develop writing is uncertain, but theirs is the oldest known writing system ‘’</p>
<p>Normally ,it should sounds like this </p>
<p>‘’''Whether the Sumerians were the first people to develop writing is uncertain, but the oldest known writing system is theirs ‘’</p>
<p>However,it is gramatically correct to put ‘‘theirs’’ after ‘‘but’’</p>