A.I. Explanation:

Option 1: "By covalent bonds between the sugar and phosphate groups."

Explanation: This option is incorrect because the text specifies that the strands of DNA are held together by hydrogen bonds between the nitrogenous bases, not by covalent bonds. Covalent bonds do exist in the structure of DNA, but they connect the sugar and phosphate groups within each nucleotide, not between the strands.

Option 2: "By ionic bonds between the nucleotides."

Explanation: This option is incorrect as the text does not mention ionic bonds as a means of holding the DNA strands together. The correct mechanism is through hydrogen bonds between the nitrogenous bases, which is a different type of bond altogether.

Option 3: "By hydrogen bonds between specific pairs of nitrogenous bases."

Explanation: This option is correct because the text explicitly states that adenine is joined to thymine by two hydrogen bonds and guanine is joined to cytosine by three hydrogen bonds. This clearly indicates that the strands of DNA are held together by hydrogen bonds between specific pairs of nitrogenous bases.

Option 4: "By peptide bonds between the adenine and cytosine."

Explanation: This option is incorrect because peptide bonds are not involved in the structure of DNA. Peptide bonds are used to link amino acids in proteins, not to connect nucleotides in DNA. The text does not support this option, as it focuses on hydrogen bonds between nitrogenous bases.