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

Chromoplasts may be of which color?

Where are thylakoids present?

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.

Which of the following is $NOT$ a food storage granule?

All plastids have similar structures because they ..........