What is the correct order of levels of organization in an organism, from the least complex to the most complex? a. organ→ system → tissue→ cell b. tissue → organ → cell → system c. system → tissue → organ → cell d. cell → tissue → organ → system

Abird flies by moving its wings up and down. which two systems are most directly responsible for the movement of a bird's wings? a. skeletal and muscular systems b. respiratory and muscular systems c. skeletal and digestive systems d. respiratory and digestive systems