public int?

? --> its simply to denote we can return null value for the Integer variable.
 
If you are not defining the integer variable with question mark, it wont accept null values. it will throw error.
 
http://msdn.microsoft.com/en-us/library/1t3y8s4s(v=vs.80).aspx
 
Refer the above link it will help you to understand the nullable types