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

Which of the following statements is incorrect?

Column-$I$ represents the organelle and Column-$II$ represents its structure. Select the correct option:

Column-$I$	Column-$II$
$(A)$ Mitochondria	$(i)$ Single membrane-bound structure
$(B)$ Lysosome	$(ii)$ Non-membrane-bound structure
$(C)$ Nucleolus	$(iii)$ Double membrane-bound structure

Which of the following statements is/are correct?
$(i)$ The shape of the cells may vary with the function they perform.
$(ii)$ Human $RBC$ is about $7.0 \; \mu m$ in diameter.
$(iii)$ Cytoplasm is the main area of cellular activities.
$(iv)$ Various chemical reactions occur in cytoplasm to keep the cell in the living state.

Which one of the following is a vital stain?

Describe the structure of the following with the help of labelled diagrams.
$(i)$ Nucleus
$(ii)$ Centrosome