Which describes a constant in an experiment?
a. the variable changed by the experimenter
b. the quantity that must remain the same
d. the variable that changes in response to the experiment
d. the statement that is worded to test the experiment

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.

the sphincter muscles at the end of the digestive tract is the anus.