人工自动免疫
参考答案：
人工自动免疫：是用人工方法接种菌苗、疫苗或类毒素等物质，刺激机体产生特异性免疫反应来获得免疫力的方法。
It is often recommended that children________chances to get close to nature.[ ]
A. are offered
B. be offered
C. should offer
D. must be offered
