PLS ANSWERRR What are the main differences between the carbon flows 300 years ago and today?

Which sentences describe disadvantages of burning fossil fuels instead of using hydrogen fuel cells? burning fossil fuels contributes to climate change. burning fossil fuels takes place inside expensive fuel cells. fossil fuels aren't widely available, but pure hydrogen is. fossil fuels are nonrenewable.

If it takes a planet 2.8 x 10^8 s to orbit a star with a mass of 6.2 x 10^30 kg what is the average distance between the planet and the star

Aa a of 22 m/s at an of 36.9 .be . a. b. c. (3)x y of? (3)go? (4) () it to to ?