Differentiate between self-pollination and cross-pollination.

$(a)$ 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?

The pistil is intact. Cross-pollination has occurred,leading to fertilization and the formation of fruit. In a bisexual flower,despite the young stamens being removed artificially,the flower produces fruit. Provide a suitable explanation for the above situation.

Which among the following statements are true for unisexual flowers?
$(i)$ They possess both stamen and pistil.
$(ii)$ They possess either stamen or pistil.
$(iii)$ They exhibit cross-pollination.
$(iv)$ Unisexual flowers possessing only stamens cannot produce fruits.

In a flower, the parts that produce male and female gametes (germ cells) are

Name the parts of a flower that serve the same function as the following parts in animals:
$(i)$ Testis
$(ii)$ Ovary
$(iii)$ Eggs
$(iv)$ Sperms