How to select two 2D objects in order to make a third object

Friend developers I am in need of your help for a simple project I am working on.
I have a collection of very simple 2d shapes like triangles and cubes. Please follow my image.

What I want is when the user clicks on object A and then on object B these objects to disappear and become one object C. After that I want the user to be able to click on another object of type A or B and object C so that these two objects disappear and become object D. This could go on indefinitely

I am stuck on how to go about this and I appreciate your help and time!

This is what I have so far

public class Selector : MonoBehaviour {
    public string FirstSelection;
    public string SecondSelection;
    public  int SelectionNum;
    // Use this for initialization
	void Start () {
        SelectionNum = 0; 


    void OnMouseDown()
        SelectionNum += 1; //selection counter

        Debug.Log("Selection " + SelectionNum);

        if (SelectionNum == 1) //if this is the first selection
            FirstSelection =; //assign the name of game object clicked
            Debug.Log("First " + FirstSelection);

        if (SelectionNum == 2) // if this is the second selection
            SecondSelection =; //assign the name of game object clicked
            Debug.Log("Second " + SecondSelection);

        // Combinations of A + A or B + A will destroy A and B gamebojects 
        if (FirstSelection == "A" & SecondSelection == "A" || FirstSelection == "B" & SecondSelection == "A")


            // Instantiate game object C (not there yet)

	// Update is called once per frame
	void Update () {

Create an empty GameObject and assign the selected Gameobjects as child to the empty gameObjects.