keep in the background的用法和样例：
- She was now in the second year of active munitions production after long covert preparations.
- The study presented here is the first step in a program aimed at reducing significantly the background count rate of NaI scintillation units.
- Cumming tittered in the background.
- A lie is plump in private but lean in public.
- The choic chanted in the background.
- They are brewing a plot behind the scenes.