自主学习：
1. reduce: v. 减少，降低，减缓
reduce pain/ costs/ speed: 减轻痛苦、降低价格、减慢速度
The price has been reduced to 50 yuan. 价格被降到50元。
by 50 yuan. 价格被降了50元。
【反义词】 increase v. 增加
2. volunteer: n. 志愿者v. 志愿做，自愿做
volunteer to do sth. 志愿做某事
Many volunteers volunteered to help the old in their town.
许多志愿者自愿帮助镇上的老人。
volunteer for sth. 志愿为某事（出力）
voluntary: adj. 志愿的 voluntarily: adv. 志愿地
