2) oxygen constantly diffuses from air or water into a animal's bloodstream. for this to happen, the concentration of oxygen in the blood must be a) greater than the concentration of oxygen in the air or water. b) greater than the concentration of carbon dioxide in the air or water. c) lower than the concentration of oxygen in the air of water. d) lower than the concentration of carbon dioxide in the air or water

Once an egg cell is fertilized by sperm, the cell then, as the embryo develops, it receives nourishment and eliminates wastes by transferring substances from its blood to its mother's blood. a. becomes a fetus immediately and exits the womb b. begins to divide and implants itself in the wall of the uterus c. remains in the uterus without dividing for several months d. travels back to the ovaries until the fetus is developed