A receptor :

Options

A. is the first portion of a reflex arc
B. initiates nerve impulses
C. responds to only one type of stimulus
D. all of the above

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the characteristics of a receptor. We need to evaluate the provided options to determine which statements about receptors are correct.

2. **Analyzing the Options**:
- **Option 1**: "It is the first portion of a reflex arc."
- This is true because receptors are indeed the first part of the reflex arc, as they detect stimuli.

- **Option 2**: "It initiates nerve impulse."
- This is also true. Receptors convert the detected stimuli into nerve impulses that are transmitted to the nervous system.

- **Option 3**: "It responds to only one type of stimulus."
- This is true as well. Different types of receptors respond to specific stimuli: mechanoreceptors respond to touch, chemoreceptors respond to chemicals, and thermoreceptors respond to temperature.

- **Option 4**: "All of the above."
- Since all the previous options are correct, this option is also true.

3. **Conclusion**: Since all options (1, 2, and 3) are correct, the correct answer is option 4: "All of the above."

**Final Answer**: The correct answer is option 4: All of the above.

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