hi, a quick question, why "at" should be written two times? see below in your example.<br /><br /><ol>
<li>Determine the critical distance downstream at which DO is at a minimum</li>
</ol><br />In the sentence, the "at" which refers to the DO is at a minimum. Thus, I thought it should be like "no at" for example, &nbsp;"Determine the critical distance downstream at which DO is&nbsp;a minimum"<br /><br />please let me know which one is correct<br /><br />thanks