根据汉语提示填空
【小题1】He has been ________________（坐牢）for nearly two years.
【小题2】I know the person you just talked with,but I cannot ________________（想起）his name.
【小题3】This medicine will ________________（保护你以防止）a return of the illness.
【小题4】When you take the underground,you must be ________________（谨防）thieves.
【小题5】Never ________________（独自外出）when it is dark.