act as most useful second的用法和样例：
- Faeces are food that has been processed by the human digestive system to extract as much useful energy as possible.
- Arabella, in what seems to be typical of Squibs, owns several cats which act as more than pets.
- Phrasal verbs, such as get up, catch up with, etc. are most useful.
- Will you act as witness to the agreement between us?