“平病”都是並母字，“田电”都是定母字，一个送气，一个不送气，说明分化的条件。
参考答案：
韵母的洪细。“平病”是洪音，“田电”是细音。
根据所给的汉语提示，转换下列句型。
1. Stamps are used for sending letters.(对画线部分提问)
_____ are stamps used_____?
2. The little boy was made to do heavy work by the boss.(改为主动语态)
The boss _____ the little boy _____ heavy work.
3. They asked me to come the next day.(改为被动语态)
I_____ ___ to come the next day.
4. The factory was built in 1956.(对画线部分提问)
__ was the factory_____?
5. So far, we haven't had any trouble.(改为同义句)
We _____had any trouble _____ now.
溶剂发泡的危害是（）。
A、增加溶剂损失
B、降低气体净化度
C、增加设备腐蚀
D、溶剂带烃
