Two chemical species X and Y combine together to form a product P which contains both X and Y. $\text{X}+\text{Y}\to \text{P}$ , X and Y cannot be broken down into simple substances by simple chemical reactions. 

Which of the following concerning the species X, Y and P are correct ?

I) P is a compound

II) X and Y are compounds

III) X and Y are elements

IV) P has a fixed composition