a. absolutely essential
s. unavoidably determined by prior circumstances
a. 必要的；必然的；必需的
1. It is necessary to study hard for the exam.
为了考试，努力学习是必要的。
2. A passport is necessary for international travel.
国际旅行需要护照。
3. It is not necessary to bring your own lunch; food will be provided.
不需要自带午餐；会提供食物。