The four most common chemical elements found in living matter are 1,C, H,O, P
2,C, H,O, N
3,C, H,N, P
4,C, H,O, S
5,C, H,O, Ca​

Explanation:

In all living system we can always find basic elements carbon, oxygen, nitrogen and hydrogen, carbon is the basic building unit contained in living matter. The percentage of carbon in the mass of living matter is 19.0% oxygen and hydrogen are present in almost all organic compound which creates living organisms.

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.