An index fossil must be A. unique in its appearance. B. smaller than other fossils around it. C. larger than other fossils around it. D. older than 1 billion years.

A.

Explanation:

Index fossils should be easily identifiable in appearance, and also geographically widespread so we can use them to "time-match" them to layers of earth that were formed around the same time, but separated at huge differences.

They kinda serve as mother nature's timestamps, so it makes sense that you want them to be easy to tell apart from other fossils from other time periods.

biological/genetic factors means they were born or predisposed to have something. nothing can really change it, and you can't really do much to "fix" it. it isn't something learned

these are the biological factors:

adrian is relatively tall and he is on the basketball team

(adrian can't ever stop being tall or force himself to be tall. likely one or both of his parents are tall as well and they passed on the tall gene. he probably does basketball because he is tall)

the rest are environmental/cultural factors. these are things that are taught to us. for example, cynthia wasn't born saving pocket money, her family most likely taught her this (environmental factor)

hope this !

are you asking for our opinion?