Choose the true statement.
a. continental crust is less dense than oceanic crust.
b. continental crust is more dense than oceanic crust.
c. continental crust and oceanic crust have the same density.

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