Facultative anaerobes :

Options

A. require a constant supply of oxygen
B. are killed in an oxygenated environment
C. do not always need oxygen
D. are photosynthetic but do not give off oxygen

Explanation

**Step-by-Step Solution:**

1. **Understanding the Term "Facultative Anaerobes":**
- The term "facultative" means optional. Therefore, facultative anaerobes are organisms that can survive with or without oxygen.

2. **Analyzing the Options:**
- **Option 1:** "Require a constant supply of oxygen" - This is incorrect because facultative anaerobes do not require oxygen all the time.
- **Option 2:** "Are killed in oxygenated environment" - This is also incorrect as facultative anaerobes can survive in oxygenated environments.
- **Option 3:** "Do not always need oxygen" - This is correct. Facultative anaerobes can use oxygen when available but can also survive without it.
- **Option 4:** "Are photosynthetic but do not give off oxygen" - This statement is inaccurate as facultative anaerobes are not necessarily photosynthetic and can produce oxygen if they perform photosynthesis.

3. **Conclusion:**
- Based on the analysis, the correct statement regarding facultative anaerobes is that they "do not always need oxygen."

**Final Answer:** They do not always need oxygen.

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