def countdown$($start$)$:

x $=$ start

if x $>0$:

return$\_$string $=$ "Counting down to $0$: $"$

while $\_\_\_$ # Complete the while loop

$\_\_\_$ # Add the numbers to the "return$\_$string"

if x $>0$:

return$\_$string $+=","$

$\_\_\_$ # Decrement the appropriate variable

else:

return$\_$string $=$ "Cannot count down to $0"$

return return$\_$string

print$($countdown$(10))$ # Should be "Counting down to $0$: $10,9,8,7,6,5,4,3,2,1,0"$

print$($countdown$(2))$ # Should be "Counting down to $0$: $2,1,0"$

print$($countdown$(0))$ # Should be "Cannot count down to $0"$