To solve the question "Choose the incorrect statement," we will analyze each option provided in the question to determine which statement is incorrect.
### Step-by-Step Solution:
1. **Analyze Option A:**
- The statement claims that nitrogenous bases (A, G, C, T, U) have heterocyclic rings.
- **Explanation:** Nitrogenous bases like adenine (A) and uracil (U) indeed have cyclic structures that contain at least two different elements (carbon and nitrogen). Therefore, this statement is correct.
2. **Analyze Option B:**
- The statement claims that DNA is the genetic material in cellular organisms.
- **Explanation:** This is true as DNA serves as the genetic material in all cellular organisms. RNA acts as genetic material only in some viruses, which are not classified as cellular organisms. Thus, this statement is also correct.
3. **Analyze Option C:**
- The statement claims that adenylic acid is a nucleoside.
- **Explanation:** Adenylic acid, also known as AMP (adenosine monophosphate), consists of a nitrogenous base (adenine), a pentose sugar (ribose), and a phosphate group. A nucleoside consists only of a nitrogenous base and a pentose sugar, without a phosphate group. Therefore, adenylic acid is a nucleotide, not a nucleoside. This statement is incorrect.
4. **Analyze Option D:**
- The statement claims that cytidine is a nucleoside.
- **Explanation:** Cytidine consists of a nitrogenous base (cytosine) and a pentose sugar (ribose), which fits the definition of a nucleoside. Therefore, this statement is correct.
### Conclusion:
After analyzing all the options, we find that the incorrect statement is **Option C: "Adenylic acid is a nucleoside."** This statement is incorrect because adenylic acid is a nucleotide, not a nucleoside.
### Final Answer:
**The incorrect statement is Option C: Adenylic acid is a nucleoside.**
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