What is a key?

Explanation

### Step-by-Step Solution:

1. **Understanding the Concept of a Key**: A key, in biological classification, is a tool used for identifying organisms based on their characteristics. It helps in distinguishing between different species or groups.

2. **Types of Keys**: There are generally two types of keys used in biological classification:
- **Dichotomous Key**: This type presents two contrasting choices at each step, guiding the user through a series of questions until the organism is identified.
- **Multi-access Key**: This allows users to choose characteristics in any order to identify the organism.

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