Give the respective scientific terms used for studying:
$(i)$ The mechanism by which variations are created and inherited,and
$(ii)$ The development of new types of organisms from existing ones.

What happens when the following situations are initiated:
$(a)$ $A$ $Planaria$ is cut into three different pieces.
$(b)$ $A$ leaf of the $Bryophyllum$ with notches falls on the soil.
$(c)$ Testosterone is released in the male reproductive system.

Why is the temperature of the scrotal sac $2\,^{\circ}C$ less than the body temperature?

Name the type of cells which undergo regeneration.

What is the placenta? Mention its role during pregnancy.

Describe sexually transmitted diseases (STDs) and mention the ways to prevent them.