Which of the following grouping below is not a group of organ systems? .
a. skeletal system, muscular system, nervous system.
b. endocrine system, digestive system, nourishing system.
c. integumentary system, excretory system, circulatory system.
d. respiratory system, immune system, reproductive system

The answer is D.

The immune system is NOT an organ system.

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.