The only two elements in alkenes and alkynes are
(1) carbon and nitrogen
(2) carbon and hydrogen
(3) oxygen and nitrogen
(4) oxygen and hydrogen
Answer: (2) carbon and hydrogen
If we look at the Chemistry Regent Reference table, we will see on Table Q, that alkenes and alkynes belong to the Homologous Series of Hydrocarbons. Hydrocarbons are organic molecules that contain carbon and hydrogen. Therefore, they must have the elements of hydrogen and carbon, making choice 2 the correct answer.
P.S: Alkenes are hydrocarbons that have a double bond. Alkynes are hydrocarbons with a triple bond.
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