There are n stairs. A person is standing at the bottom and he wants to reach the top. The person can climb either 1,2, or 3 stairs at a time. The function below counts the number of ways that the person can reach the top.

def stairs(n):

if n <= 1: return n

else:

_______________ <-- line 5

Which of the following should be placed in line 4?

a)return stairs(3)

b)return stairs(n-1)

c)return stairs(n-1) + stairs(n-2)

d)return stairs(n-1) + stairs(n-2) + stairs(n-3)