What was the key observation in the Meselson and Stahl experiment after the second round of DNA replication?

• A. DNA with an intermediate density and DNA with light density
• B. DNA with intermediate density, consisting of one strand of ¹⁵𝑁 and one strand of ¹⁴N
• C. DNA molecules containing only ¹⁴𝑁 strands
• D. DNA molecules containing only ¹⁵𝑁 strands

Answer: (A)