A couplet in a key is

Options

A. each statement in the key
B. contrasting characters in a pair
C. rejection of a statement
D. none of these

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the definition of a "couplet" in the context of a taxonomic key. A taxonomic key is a tool used for identifying organisms based on their characteristics.

2. **Definition of a Taxonomic Key**: A taxonomic key consists of a series of couplets. Each couplet provides two contrasting descriptions of an organism's features.

3. **Identifying the Characteristics of a Couplet**: A couplet is defined as a pair of opposing descriptions that help the user make a choice about the characteristics of the organism they are trying to identify.

4. **Analyzing the Options**:
- **Option 1**: "Each statement in the key" - This is incorrect because a couplet specifically refers to a pair of statements, not just one.
- **Option 2**: "Contrasting characters in a pair" - This is correct as it accurately describes the nature of a couplet.
- **Option 3**: "Rejection of a statement" - This is incorrect because a couplet does not refer to rejection but rather to a choice between two descriptions.
- **Option 4**: "None of these" - This is incorrect since we have identified a correct option.

5. **Conclusion**: The correct answer is "contrasting characters in a pair," as this accurately describes what a couplet is in a taxonomic key.

**Final Answer**: Contrasting characters in a pair.

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