Twins produced when a single egg is fertilized are called ______________twins. Twins produced from two eggs ovulated at the same time are called ______________ twins.

A. Sperm; semen: Sperm is the male gamete, and semen is the fluid that contains sperm, but these are not analogous to the ovary, which is a gonad.

B. Testis; semen: The testis is the male gonad that produces sperm, but semen is the fluid that contains sperm. The analogous structure to the ovary is the testis, but this pairing with semen is not correct.

C. Semen; sperm: Semen is the fluid containing sperm, while sperm is the male gamete. Neither is a gonad, so this pairing is not analogous to the ovary.

D. Testis; sperm: The testis is the male gonad that produces sperm, making it analogous to the ovary, which is the female gonad that produces eggs.

A. Ductus venosus and ductus arteriosus: The ductus venosus bypasses the liver, not the lungs. The ductus arteriosus does allow blood to bypass the lungs by connecting the pulmonary artery to the aorta. However, this option is not entirely correct since the ductus venosus is not involved in bypassing the lungs.

B. Foramen ovale and ductus arteriosus: The foramen ovale allows blood to pass directly from the right atrium to the left atrium, bypassing the lungs. The ductus arteriosus connects the pulmonary artery to the aorta, also bypassing the lungs.

C. Fossa ovalis and umbilical arteries and vein: The fossa ovalis is a remnant of the foramen ovale in adults and does not function in fetal circulation. The umbilical arteries and vein are involved in exchanging oxygenated and deoxygenated blood between the fetus and placenta, not bypassing the lungs.

D. Ligamentum venosum and ligamentum arteriosum: These are the adult remnants of the ductus venosus and ductus arteriosus, respectively. They do not play a role in fetal circulation.