"suppose you hold a 10 pound weight steadily in your right hand with your elbow slightlly bent, so that you maintain a contraction of your biceps brachii muscle. after some time, you experience pain and it becomes increasingly difficult to keep the weight at the same level. explain why this is so."

After holding the weight for so long, the muscle would get tired, in an effort to tell the brain to stop holding the weight because the muscle fibers cannot handle it any more. Lactic acid is a by-product during muscle contraction that leads to the pain.

significant amounts of nitrogen oxides, carbon monoxide, and other pollution

explanation:

passenger vehicles are a major pollution contributor, producing significant amounts of nitrogen oxides, carbon monoxide, and other pollution. in 2013, transportation contributed more than half of the carbon monoxide and nitrogen oxides, and almost a quarter of the hydrocarbons emitted into our air.