Sperm must travel to the distal end of the uterine tube to encounter the egg before it dies.

A. Ca2+: prevents the entrance of any more sperm- While calcium ions play a role in the slow block to polyspermy, the fast block involves sodium ions.

B. Na+: prevents the entrance of more Na- The fast block to polyspermy involves sodium ions (Na+), but the depolarization of the egg membrane prevents the entrance of more sperm, not just more Na+.

C. Na+: prevents the entrance of Ca2+. The fast block prevents additional sperm from entering, not calcium ions.

D. Na+: prevents the entrance of any more sperm: The fast block to polyspermy involves the opening of sodium ion channels, leading to depolarization of the egg membrane. This depolarization prevents other sperm from binding to and penetrating the egg.

A. Small intestine is the primary site of nutrient absorption. The majority of digestion and absorption of nutrients such as carbohydrates, proteins, and fats occurs in the small intestine, particularly in the duodenum and jejunum.

B. Pancreas secretes digestive enzymes into the small intestine but does not absorb nutrients itself.

C. Stomach plays a role in the digestion of proteins and the mechanical breakdown of food, but it absorbs very few nutrients, primarily alcohol and some drugs.

D. Large intestine absorbs water and electrolytes, but it is not the major site of nutrient absorption. Most nutrients are absorbed before food reaches the large intestine.