Answer:
d. They always invade the human body from the external environment
Explanation:
Antigens are the molecules that have the ability to stimulate immune response and are mostly protein in nature. However, an antigen may be self or non-self molecule.
This means that antigen may enter the human body from the external environment (the non-self antigen) or any molecule present inside the body may serve as antigen (self-antigen). Self-antigens are responsible for autoimmune disorders.
Number of organisms per unit time.
A. endoskeleton
Endoskeleton is not a characteristic of Arthropods that has attributed to their diversity and success. Endoskeleton is defined as inner characteristics of the skeletal feature of an organisms such as bears, humans, fish and etc.
Exoskeleton on the other hand is the trait by which arthropods posses. It is skeletal feature that is described outside or is extrinsic, this is evident as crabs have their skeletons outside of their body.
nucleus is the structure responsible.
The answer is B i did this before hope this helps