What does the corpus luteum secrete?

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 best definition of the process of gastrulation is that it is a process where the

In humans,the foetal membrane which forms the intimate connection with the uterine tissue is:

How many of the following hormones are produced only during pregnancy in a woman?
Estrogen,Progesterone,Relaxin,$hCG, hPL$

Which of the following participate in the formation of the placenta?