In which stage is the blastocoel formed?

After one month of pregnancy,the embryo's $A$ is formed. By the end of the $B$ month of pregnancy,the foetus develops limbs and digits. By the end of $C$,most of the major organ systems are formed,for example,the limbs and external genital organs are well-developed. By the end of $D$,the body is covered with fine hair,eyelids separate,and eyelashes are formed. Here $A$ and $D$ refer to:

The average duration of human pregnancy is approximately ........... months.

What is a twin?

Which hormone is produced only in the female body during pregnancy by the placenta?

Which of the following statements is true regarding cleavage?