Passing Derived class as a parameter to a method when the parameter type is base class

I am a newbie and trying to understand concepts of inheritance and design patterns.

I came across this pattern http://en.wikipedia.org/wiki/Strategy_pattern when I was going through some blog.

I found it interesting and wanted to learn more. So I developed the following program.

static void Main(string[] args)
{
    Context context;

    // Three contexts following different strategies
    context = new Context(new ConcreteStrategyAdd());
    int resultA = context.executeStrategy(3, 4);

    context = new Context(new ConcreteStrategySubtract());
    int resultB = context.executeStrategy(3, 4);

    context = new Context(new ConcreteStrategyMultiply());
    int resultC = context.executeStrategy(3, 4);

    Console.Read();
}      

abstract class Strategy
{
    public abstract int execute(int a, int b);

    public void Test()
    {
        Console.Write("tttt");
    }
}

class ConcreteStrategyAdd : Strategy
{
    public override int execute(int a, int b)
    {
        Console.WriteLine("Called ConcreteStrategyAdd's execute()");
        return a + b;  // Do an addition with a and b
    }
}

class ConcreteStrategySubtract : Strategy
{

    public override int execute(int a, int b)
    {
        Console.WriteLine("Called ConcreteStrategySubtract's execute()");
        return a - b;  // Do a subtraction with a and b
    }
}

class ConcreteStrategyMultiply : Strategy
{
    public override int execute(int a, int b)
    {
        Console.WriteLine("Called ConcreteStrategyMultiply's execute()");
        return a * b;   // Do a multiplication with a and b
    }
}

class Context
{
    private Strategy strategy;

    // Constructor
    public Context(Strategy strategy)
    {
        this.strategy = strategy;
    }

    public int executeStrategy(int a, int b)
    {
        return strategy.execute(a, b);
    }
}

The program compiles fine and is workin