ANSWER:
C. Red blood cell, heart tissue, circulatory system
BECAUSE:
Red bloods cells make up heart tissues and heart tissues make up the organs in the circulatory system.
Red bloods cells are therefore simpler than heart tissues and heart tissues simpler than the circulatory system.
It can’t be A because blood is simpler than the circulatory system
It can’t be B because nerves and the brain are simpler than the nervous system
It can’t be D because bone is simpler than the skeletal system
