Answer 1

Nullable types are instances of the System.Nullable<T> struct. A nullable type can represent the correct range of values for its underlying value type, plus an additional null value. For example, a Nullable<Int32>, pronounced "Nullable of Int32," can be assigned any value from -2147483648 to 2147483647, or it can be assigned the null value
bro you could googke this things
https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/nullable-types/