How many elements are needed to make a compound?

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.

The air that we breath is made mostly of which gaseous molecule

What is the coefficient of kinetic friction μk between the block and the tabletop?