A recursive in-place array reverse function

We wanted to have a function that could reverse a string in-place like this:

char str[] = "ABCDEF";
Reverse(str, strlen(str));

So we implemented this generic recursive Reverse() function:

template<typename T>
void Reverse(T* arr, size_t len) {
  if(len > 1) {
    std::swap(arr[0], arr[len-1]);
    /* Reverse(??, ??); */
  }
}

The recursive call in above code is omitted for you to fill in. Which one of followings should replace the comment in above code to complete the implementation of Reverse()?