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 = gameObject.name; //assign the name of game object clicked
             Debug.Log("First " + FirstSelection);
         }
 
         if (SelectionNum == 2) // if this is the second selection
         {
             SecondSelection = gameObject.name; //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")
 
             {
            
             
             Destroy(GameObject.Find("A"));
             Destroy(GameObject.Find("B"));
 
             // Instantiate game object C (not there yet)
 
         }
             
             
         
     }
     
     
     
     // Update is called once per frame
     void Update () {
     
     }
 }