Give two reasons for the appearance of variations among the progeny formed by sexual reproduction.

State whether the following statement is true or false: $A$ thick and spongy lining of the uterus is required for nourishing the embryo if fertilization has taken place.

$(a)$ Name the parts labelled as $A, B, C$ and $D$ in the diagram given below:
$(b)$ What is pollination? State its significance.
$(c)$ How does fertilization occur in flowers? Name the parts of the flower that develop into $(i)$ seed,and $(ii)$ fruit after fertilization.

Which of the following organisms undergoes binary fission?

What are those organisms called which bear both the sex organs in the same individual? Give one example of such an organism.

What are sexually transmitted diseases? Name an $STD$ which damages the immune system of the human body.