What’s the following have contributed to historical increases in the global human population? A.) increases in the number of deaths
B.) unavailability of clean water
C.) decreases in the birth rate
D.)Advances in hygiene

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