Which of the following is not found in the embryo sac of a dicotyledonous plant?

Megasporangia arise from the ....

In an embryo sac of a typical angiosperm,there are:

The three cells present at the chalazal end form $.....P......$,and the three cells present at the micropylar end form $.....Q.....$.
$\quad\quad\quad \quad \text { P } \quad\quad\quad \text { Q }$

Horizontally oriented ovules are called:

In the given figure,what do 'a','b',and 'c' represent respectively?