预防接种
参考答案：
又称为人工免疫，是将生物制品接种到人体内，使机体产生对某种传染病的特异性免疫力，以提高人群免疫水平，预防传染病的发生与流行。
Read and choose.(给下面的图片选择合适的描述。)
1. He is at school now. He will be home at four o'clock.
2. Father is at the office now. He will be home at six o'clock.
3. Maomao is on the train. She will be home at nine o'clock.
4. Lily is in the park now. She will be home at seven o'clock.
