a facultative course的用法和样例：
- For example, a facultative anaerobe is an organism that can grow under anaerobic conditions but is also able to survive in aerobic conditions.
- Legionella pneumophila, the most common cause of Legionaires' disease, is a facultative intracellular parasite bacterium that causes epidemic, sporadic, and nosocomial pneumonia.
- He go on a refresher course in bookkeeping.
- It's a facultative enactment.