There are 20 mice in a 5 m2 field ecosystem. The population density of mice is

D

Explanation:

The population density would be 20 / 5 = 4 mice per m².

the core of the sun is so hot and there is so much pressure, nuclear fusion takes place, hydrogen is changed to helium. nuclear fusion creates heat and photons. when the heat reaches the earth it warms the molecules of the atmosphere, and they warm other molecules and so on.