What is the reason for the growth of the pollen tube towards the ovule?

$(a)$ List two reasons for the appearance of variations among the progeny formed by sexual reproduction.
$(b)$ $(i)$ Name the part marked '$A$' in the diagram.
$(ii)$ How does '$A$' reach part '$B$'?
$(iii)$ State the importance of the part '$C$'.
$(iv)$ What happens to the part marked '$D$' after fertilisation is over?

In the given figure,the parts $A, B$,and $C$ are sequentially:

In the figure,the parts $A$,$B$,and $C$ are sequentially:

Which part of the plant is the reproductive organ?

Distinguish between pollination and fertilisation. Mention the site and product of fertilisation in a flower.
Draw a neat,labelled diagram of a pistil showing pollen tube growth and its entry into the ovule.