Final answer:
The trophoblast cells secrete hCG to maintain the corpus luteum during pregnancy.
Step-by-step explanation:
During pregnancy, the trophoblast cells secrete human chorionic gonadotropin (hCG) to maintain the corpus luteum while the placenta is being built. The hCG is secreted by the trophoblast cells near letter B. The hCG directs the corpus luteum to produce progesterone and estrogen which are necessary for the developing embryo.