Which of these choices are an example of a common?
1) a dairy cow
2) a service animal
3) a wild duck
4) a pet cat

4) A pet cat

Explanation:

Common can be define as the similarities in things, living beings, behavior, forms, shapes and others.

A pet cat is a domestic animal which can be found in many houses. Hence, it can be considered as common among human beings. A dairy cow cannot be kept by all although it is also a domestic animal. A service animal is also not found commonly everywhere. A wild duck can be found were a pond or river exists and it is also not common for all as it not kept as domestic pet.

A pet cat

Explanation:

his research revealed two genetically different groups of prokaryotes, which justified splitting kingdom monera into two kingdoms. the genetic and cellular differences were so great, that these two kingdoms were eventually placed into different domains in the current three-domain system.