Solve for t - r:
s - r = t - s
2s = t + r
2s - 2r = t + r - 2r=t - r
2(s-r)=t-r
Since w - v = t - s, (w - v) + (t - s) = 2(w - v). And
since w - v = s - r, 2(w - v) = 2(s - r), which is also equal to t - r.
The answer is C because t - r = (w - v) + (t - s).