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

What is common between chloroplasts,chromoplasts,and leucoplasts?

All plastids have essentially the same structure because

Which of the following statements are correct?
$(A)$ The number of chloroplasts varies from $1$ per cell in $Chlamydomonas$, a green alga, to $20-40$ per cell in the mesophyll.
$(B)$ Out of the two membranes of chloroplasts and mitochondria, the inner membrane is relatively less permeable.
$(C)$ Thylakoids are arranged in stacks like the piles of coins called grana (singular: granum) or the intergranal thylakoids.
$(D)$ The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins.

In the given figure,what does $B$ represent?

Match the following columns:

Column-$I$	Column-$II$
$P$. Amyloplast	$I$. Fats
$Q$. Elaioplast	$II$. Starch
$R$. Aleuroplast	$III$. Protein