Since you use a string to return another string, using a
Dictionary<string, string>
would be pretty straightforward, e.g.
Dictionary<string, string> response = new Dictionary<string, string>()
{
{"Tell me a joke", "Why did the chicken cross the road?"},
{"I don't know", "to get to the other side"},
{" ", "Take a Guess"},
};
Console.WriteLine("The answer to {0} is {1}", "I don't know", response["I don't know"]);