Fill in the code to complete the following function for computing a Fibonacci number. Please select all that apply.

def fib$($index$)$:

if index $==0$ : # Base case

return $0$

elif index $==1$: # Base case

return $1$

else: # Reduction and recursive calls

return

fib$($index $-1)$

fib$($index $-2)$

fib$($index $-1)+$ fib$($index $-2)$

fib$($index $-2)+$ fib$($index $-1)$