<h2>The answer to 9 is <em><u>slippage of a block of material moving along a curved surface</u></em></h2>
A........................
Valves ensure blood flows toward your heart. Vein valves work to bring the blood that flowed down via arteries back up to your heart.
The correct answer is (a) driving.
The pressence of undesirable substances in air is called as air pollution. There are many human activities that contribute to air pollution. The biggest contributor is driving. The pollutants that comes out of the car are carbon dioxide, carbon monoxide. They also release hydrocarbon and nitrogen oxides which when combines in sunlight becomes ozone. Air pollutants emitted by cars and other automobiles increases the level of harmful gases which to severe health problems such as heart disease, asthma, irritation in eyes.
