Double fertilization was discovered by . . . . . . .

Double fertilization is the fusion of

Which cell of the female gametophyte is involved in the formation of primary endosperm nucleus $(PEN)$ after fertilisation?

What is double fertilization?

Number of nuclei taking part in double fertilization is

In angiospermic fertilisation,one male gamete fuses with the egg to form $A$,and this event is called $B$. Identify $A$ and $B$ and choose the correct option.