How do the bases guanine and adenine differ from each other, given that both are purine bases?

• A. Guanine has a keto group at position 6, while adenine has an amino group.
• B. Guanine has a keto group at position 6 while adenine has an amino group at position 2.
• C. Guanine has a single ring in its structure, while adenine has a fused two-ring system.
• D. Guanine lacks a keto group at position 6, while adenine has such a group.

Answer: (A)