During my leaning training, I came across this question when reading this sentence in my English workbook.

> 
> a) What an interesting program this is!!
> 
> 
> 

I wonder why **"a"** is used?? Why the answer book has only this sentence? 
Why If I use **"the"** instead of **"a"** (like in the sentence below), that is wrong? 

> 
> b) What the interesting program this is!!
> 
> 
> 

I couldn't find any references in my study-aid book. 
But I think I can use **"the"** in this sentence because using **"a"** implies that I don't know the program. So I think there is a possibility to use **"the"**.

Please tell me is my thought wrong or not?