Oxygen & hydrogen this may be the answer
B) Embryonic stem cells research requires the use of human embryos.
The ileum contains villi that project into the lumen and increase the amount of surface area.
Villi are small finger-like structures that project into the lumen of the small intestine. Villi increase the surface area of the intestinal walls for easy and quick absorption of digested food with the addition of digestive secretions. Villi vary in length from about 0.5 to 1 mm. They are usually found in large numbers at the beginning of the small intestine and they reduce toward the end of the tract.
The combination of drought and poor land use practice created the the dust bowl.
I would say A because if the ball is rolling down the hill the it won't stop till it hits the bottom.
Hope This Helps! Have A Nice Day!!