A drunkard is walking along a straight road. He
takes 5 steps forward and 3 steps backward and so
on. Each step is 1 m long and takes 1 s. There is a
pit on the road 11 m away from the starting point.
The drunkard will fall into the pit after
A 21 s
B 29 s
C 31 s
D 37 s.