.......................... in RNA replaces thymine in DNA.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Components of Nucleotides**:
- Nucleotides are the building blocks of nucleic acids (DNA and RNA) and consist of three components: a nitrogenous base, a pentose sugar, and a phosphate group.

2. **Identifying the Sugars**:
- In RNA, the sugar present is ribose, while in DNA, the sugar is deoxyribose. This difference is crucial in distinguishing between the two nucleic acids.

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