OrderDynamics Senior .NET Developer/Architect interview questions

The interviewer asked .Net C# related questions mostly and some relational database questions. 1. What are abstract classes? 2. What are interfaces? 3. What are static classes? Can we instantiate an instance of these classes? 4. What are nullable types? 5. Do C# support multiple inheritance? 6. What is polymorphism? 7. What are indexes? Why do we need them? 8. Why do we use group by clause? 9. Have you worked using Design Patterns? 10. Name any two design patterns and for what purpose they are used?.