Reverse an alien invasion programming exercise

Reverse an alien invasion, programming exercise.

There is an alien invasion in the city of the Young letters. The aliens are also characters, but somehow different. Help the citizen restore peace by writing this function (they offer to make you the mayor):

In a given string, reverse the order of the string but first remove all non-alphabetic characters.

Example:
"abc" => "cba"
"ab!@#11235467c" =>"cba"

    public static string ReverseAndRemove(string str)
    {   
        //this is a small and simple program.
        //First we will reserve some space for the result
        string r = "";
        //Then we will go through each character and
        //save all the letters in our string "r"
        //The trick to determine if char is a letter or not
        //is to check its ASCII code number. 
        for (int i = str.Length - 1; i >= 0; i--)
        {                
            if (str[i] >= 'A' && str[i] <= 'z')
            {
                r += str[i].ToString();
//or you can try what this line of code do:
//if (char.IsLetter(str[i])) Console.WriteLine(str[i]);
            }               
        }               
        return r;
    }