Generics - General Questions

What is the output of the following program?

var d = new Dictionary<string, int> { ["a"] = 1 };
d["a"] += 4;
Console.WriteLine(d["a"]);

Which constraint requires T to be a reference type?

What is the output of the following program?

var nums = new List<int> { 4, 9, 2 };
Console.WriteLine(nums.Contains(9) + " " + nums.IndexOf(2));

What does the constraint 'where T : new()' require?

Generic type parameters are conventionally named: