What is one reason we absolutely need CO2 in Earth’s atmosphere?

The Short
Carbon is in carbon dioxide, which is a greenhouse gas that works to trap heat close to Earth. It helps Earth hold the energy it receives from the Sun so it doesn't all escape back into space. If it weren't for carbon dioxide, Earth's ocean would be frozen solid.

The answer is C. observations

Scientists often conduct questions off of observations and then create a hypothesis leading to the conclusion.