Why Do Leaves Have Internal Air Spaces

Why do leaves have internal air spaces?

A) to allow all the mesophyll cells to evaporate water and thus pull nutrients to them via an evaporatively driven flow
B) because CO₂ diffusion is 10,000 times faster in air than in water
C) to make leaves lighter and less dense
D) because O₂ diffusion is 10,000 times faster in air than in water