Which one of the following statements is correct?

• A. Adenine forms two hydrogen bonds with guanine; thymine forms three hydrogen bonds with cytosine.
• B. Adenine forms three hydrogen bonds with thymine; guanine forms two hydrogen bonds with cytosine.
• C. Adenine forms two covalent bonds with thymine; guanine forms three covalent bonds with cytosine.
• D. Adenine forms three covalent bonds with thymine; guanine forms two covalent bonds with cytosine.
• E. Adenine forms two hydrogen bonds with thymine; guanine forms three hydrogen bonds with cytosine.