Given that the variable my_string has been initialized to some non-empty string, select all valid lines of code that will access the last letter in my_string my_string[len(my_string)] my_string[:len(my_string)] my_string[len(my_string)-1:len(my_string)] Omy_string[0] my_string[-1] my_string[len(my_string)-1] my_string[len(my_string):len(my_string)] Omy_string[len(my_string)-1:]