Read the dialogue and answer the questions carefully!

Mother: What do you think of this refrigerator, girl? it's good, isn't it?

Laura: I don't quite think so, Mom. I think the other one is better.

Mother: But this one is cheaper. And look at the design. It's great, isn't it?

Laura: Yes, but it doesn't have an Energy Star sticker on it.

Mother: Really? What does a sticker affect?

Laura: Well, the sticker tells you that the product has been checked and approved by an international body of its efficiency. All electronic appliances which have this sticker on it mean they don't waste electricity. So, not having that sticker on may mean this fridge must be requiring much energy to operate. I think we'd better choose the fridge with this sticker so that we can automatically save money from saving the electricity.

Mother:  Are you sure?

Laura: I'm definitely sure, Mom.

Mother: Where did you learn that from?

Laura: From an article that I read before.

Mother: That's good. OK, we'll take the one with the Energy Star sticker on it.

What kind of expression is the underlined sentence?