A runner is initially at rest at the starting line of a $100$m race. When he hears the starting gun, he accelerates at a constant rate of ai$=0\mathrm{.}55$m$/$s$,$ which he maintains until he crosses the finish line at time t$=$t$1.$ It then takes him an additional $5\mathrm{.}5$ s to come to a complete stop at time t$=$t$2.$ Use the given coordinate system where x is positive to the right. 

 A: Enter an expression for the time, t$1,$ that it takes for the runner to cross the finish line in terms of d and ai$.$ 

 B: Enter an expression for the runners speed, vi$,$ when he crosses the finish line in terms of ai and t$1$ only. 

 C: Calculate the runners constant acceleration, in meters per second squared, after he crosses the finish line until he comes to rest.