Consider a recursive function called palind romeR $(s$ tr $)$ that returns true if a string is a palindrome $($reads the same backwards and forwards$).$

What is a valid base case?

len$(str)==0$

$str==\text{'}$

len$(str)1$

$3$ tr $==$ None

$4$

$1,2,$ and $3$

All of the cases are valid

None of the cases are valid

$1$ and $2$