Why does the lining of the uterus become thick and spongy every month?

What is placenta? Describe its structure. State its functions in case of a pregnant human female.

For approximately how many days does menstruation last in human females?

How are gametes formed in sexually reproducing organisms?

In the diagram of the human male reproductive system given below:
$(a)$ Label parts $A$ and $B$.
$(b)$ Name the hormone produced by organ $X$. What is the role of this hormone in the human male?
$(c)$ Mention the name of substances that are transported by tubes:
$(i)$ $C$ and $(ii)$ $D$.

During adolescence,several changes occur in the human body. Mark one change associated with sexual maturation in boys.