The and systems work together to support the body and enable it to move.
a) skeletal and muscular
b) muscular and endocrine
c) lymphatic and skeletal
d) circulatory and respiratory

A) skeletal and muscular

Explanation:

The muscles are connected to the bones, this connection collectively forms a musculoskeleton system. The primary function of this system, includes the movement of the body, other functions include: providing stability during standing, walking and running, maintaining posture and providing protection against injury.

The joint is the point of articulation of two bones. The tendons are the connective tissues which connect muscle to bone. The ligaments are the connective tissues which connect bones to bones. Both of these connective tissues helps in flexible movement of the muscles and the bones.

when it is at an intermediate population density