<p>I know the book has an explanation but it's unclear... </p>
<p>The answer in the book is (A) None, but I think it could be (C), III only...</p>
<p>Can't <T be directly below <S and directly right of <R? This would mean the slope of RT is zero and the slope of ST is undefined... Does undefined work in a sum (count as zero)? The sum would be und.+0+1... does that equal 1?</p>
<p>Thanks to anyone who takes the time to answer this.</p>