What kind of variation is exhibited by human bloods?

Blood provides an ideal opportunity for the study of human variation without cultural prejudice.  It can be easily classified for many different genetically inherited blood typing systems.  Also significant is the fact that we rarely take blood types into consideration in selecting mates.  In addition, few people know their own type today and no one did prior to 1900.  As a result, differences in blood type frequencies around the world are most likely due to other factors than social discrimination.  Contemporary Japan is somewhat of an exception since there are popular Japanese stereotypes about people with different blood types.  This could affect choice in marriage partners for some Japanese. All human populations share the same 29 known blood systems, although they differ in the frequencies of specific types.  Given the evolutionary closeness of apes and monkeys to our species, it is not surprising that some of them share a number of blood typing systems with us as well. When we donate blood or have surgery, a small sample is usually taken in advance for at least ABO  and Rh  systems typing.  If you are O+, the O is your ABO type and the + is your Rh type.  It is possible to be A, B, AB, or O as well as Rh+  or Rh- You inherited your blood types from your parents and the environment in which you live cannot change them. I took it from a website: http://anthro.palomar.edu/vary/vary_3.htm

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.