Fill in the blanks:

$\left(\mathrm{a}\right)$ By the end of the ___a__ of pregnancy, the fetus develops limbs and digits.

$\left(\mathrm{b}\right)$ By the end of ___b__ the body is covered with fine hair, eyelids separate and eyelashes are formed.

$\left(\mathrm{c}\right)$ After ___c__ of pregnancy, the embryo's heart is formed.