Q.  You know that the extension of a private telephone number is 343, but you have forgotten the last 4 digits.  You can only recall that the last 4 digits are 3, 6, 8, and 9, but you do not know the order.  What is the maximum number of telephone calls you will need to make in order to dial the correct number? 

1. 20
2. 24
3. 18
4. 4