Operators - General Questions

What is the output of the following program?

int? n = null;
n ??= 25;
Console.WriteLine(n);

What is the output of the following program?

int a = 10;
a += 5;
a *= 2;
Console.WriteLine(a);

What is the output of the following program?

Console.WriteLine(1 << 4);

What is the output of the following program?

string s = null;
string t = s ?? "default";
Console.WriteLine(t);

What does the 'is' operator do?