Convection . 1.occurs in fluids
2. requires physical contact between molecules
3.occurs in solids
4. is the method of heat transfer used by the Sun

1. occurs in fluids

Explanation:

the movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat.

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.