The normal or $Polygonum$ type of embryo sac is

The ovule is morphologically equivalent to:

In the embryo sac,after which stage does cell wall formation occur?

The largest cell of the mature embryo sac is

Given below are two statements: One is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A):$ $A$ typical unfertilised,angiosperm embryo sac at maturity is $8$-nucleate and $7$-celled.
Reason $(R):$ The egg apparatus has $2$ polar nuclei.
In the light of the above statements,choose the correct answer from the options given below:

How many nuclei in the embryo sac are surrounded by cell walls and organized into cells?