How is an ionic bond formed?
an electron is transferred from one atom to another.
two atoms share the electrons in their outermost shells.
both atoms in the bond lose an electron.
both atoms in the bond gain an electron.

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 correct option is option d