The encounter occurred as a result of european
explorers crossing the
(1) atlantic ocean
(2) sahara desert
(3) andes mountains
(4) mediterranean sea

The encounter occurred as a result of European explorers crossing the Atlantic Ocean in 15th century. They unexpectedly met native Americans.

Air pollution: “combustion results in gases such as nitrogen oxides, nitrogen dioxide and particulates. the main causes of air pollution include transport, domestic combustion and industrial processes. pollutants from these sources can cause problems in the immediate vicinity and a distance away.”

Answer:   [b]:   "cultural" .