How is a dichotomous key used?
a. the organism is compared to paired pictures of organisms until a match is found in the key.
b. the organism’s dna is sequenced and compared to the dna sequences that are listed in the dichotomous key.
c. observable characteristics of the organism are checked through a sequence of two-part steps until an identification is listed in the key.
d. observable characteristics of the organism are listed in a group and compared to different groups of observable characteristics listed in the key.

The right option is; C. Observable characteristics of the organism are checked through a sequence of two-part steps until an identification is listed in the key.

A dichotomous key is a tool created by scientists to help its users to reliably identify unknown organisms (plants and animals) and objects. A dichotomous key usually contain series of questions which are given in two choices in each step. Physical characteristics of the unknown organism are checked and used to answer the questions. When a question is correctly answered, it directs the user to the next one until the organism is correctly identified with its scientific name in the key.

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.