I honestly think it is D. Impulses.
Explanation:
that's it. you are on course as shown on the screen
The correct answer is antigens and epitope.
An antigen is a substance which triggers an immune response in an organism. This can be either a foreign substance, such as a pathogen, or a specific part of the host organism (in this case this is an autoimmune response). The presence of an antigen activates a specific part of the immune system, called antibody. Each antigen has a specific antibody, which is tailored by the immune system accordingly. More specifically, the antibody has a specific structure, called paratope, which is complementary and binds like a key to a specific structure of the antigen, called epitope.
The acetabulum is the cup-like structure where the femur articulates. The head of the femur is the specific structure which articulates in the acetabulum.
