The two strands of DNA are held together by bond`:`

Options

A. Nitrogen
B. Oxygen
C. Hydrogen
D. Carbon.

Answer

### Step-by-Step Solution:

1. **Understanding DNA Structure**:
- DNA (Deoxyribonucleic Acid) consists of two strands that form a double helix structure. Each strand is made up of nucleotides, which include a sugar, a phosphate group, and a nitrogenous base.

2. **Identifying the Bonds**:
- The two strands of DNA are complementary and run in opposite directions (antiparallel). The nitrogenous bases from each strand pair up to connect the two strands.

3. **Types of Nitrogenous Bases**:
- The nitrogenous bases in DNA include adenine (A), thymine (T), cytosine (C), and guanine (G).
- Adenine pairs with thymine, and cytosine pairs with guanine.

4. **Nature of the Bonds**:
- The connection between the nitrogenous bases of the two strands is not a covalent bond but rather a weaker interaction. This type of bond allows the strands to separate during DNA replication and transcription.

5. **Identifying the Correct Bond**:
- The specific type of bond that holds the nitrogenous bases together between the two strands of DNA is called a hydrogen bond.
- Adenine and thymine are held together by two hydrogen bonds, while cytosine and guanine are held together by three hydrogen bonds.

6. **Conclusion**:
- Therefore, the correct answer to the question "The two strands of DNA are held together by bond:" is **hydrogen bond**.

### Final Answer:
The two strands of DNA are held together by **hydrogen bonds**.

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