Which of the following features are correct? Choose correct option. a. Semiconservative DNA replication was experimentally proven by Meselson and Stahl. b. They used 15N as a heavy isotope and 14N as a light isotope of nitrogen. c. After one generation, the DNA molecules were hybrid (15N14N). d. After two generations, the DNA molecules were 14N14N and 15N14N in equal proportions.