Out of the 200 million ejaculated sperm, only about _________ reach the vicinity of the egg.

A. the embryo; part of the placenta. The embryoblast (inner cell mass) will become the embryo, but the trophoblast will become part of the placenta, not the embryo.

B. part of the placenta; the embryo. The trophoblast forms part of the placenta and helps in its development, while the embryoblast (inner cell mass) will develop into the embryo.

C. part of the placenta; the yolk sac. While the trophoblast does contribute to the formation of the placenta, the embryoblast (inner cell mass) develops into the embryo, not the yolk sac. The yolk sac is derived from extraembryonic mesoderm and endoderm, not the inner cell mass directly.

D. the embryo; the yolk sac. The embryoblast (inner cell mass) develops into the embryo, but the trophoblast does not become the yolk sac. The yolk sac is formed from structures surrounding the embryo, not from the trophoblast itself.

A. Tubuloglomerular feedback involves the macula densa sensing changes in sodium chloride concentration and adjusting the glomerular filtration rate (GFR) via the juxtaglomerular apparatus.

B. Renal autoregulation includes mechanisms like the myogenic mechanism and tubuloglomerular feedback that maintain stable GFR despite blood pressure changes. This option is too broad and not specific to the mechanism described.

C. The myogenic mechanism refers to the smooth muscle's tendency to contract when stretched, helping to stabilize GFR by adjusting the diameter of the afferent arterioles. This is the correct answer.

D. Sympathetic control influences renal blood flow and GFR through hormonal and nervous system regulation, but it is not the specific mechanism described in the question.