题目

短文改错It was extreme cold last Monday morning. While I was walking on my way to school, I noticed two street cleaner sweeping the street. Seeing them work so hard on such a cold morning, I was moved and made up my mind do something for them. After arriving at school, I discussed with some classmates that we could do. The next day, they got up early and prepared porridge (粥) at home. When we handed the cleaners with the hot porridge and expressed our appreciation for their hard work, they looked a little surprising at first. And then they takes the porridge and thanked us happily. Since蒸馏水和澄清石灰水分别长期露置在空气中，它们的PH变化是（　　）A．两者都不变B．前者变小，后者变大C．两者都变小D．前者变大，后者变小