What is a Primary energy source

Primary energy consists of unconverted or original fuel

Explanation:

for example, primary energy sources include petroleum, natural gas, coal, biomass, flowing water, wind, and solar radiation

Those are the fuels that can be mined, reaped, extracted, harvested, or harnessed directly

burning of wood and coal

explanation:

the panel also concluded there's a better than 95 percent probability that human-produced greenhouse gases such as carbon dioxide, methane and nitrous oxide have caused much of the observed increase in earth's temperatures over the past 50 years.

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.