根据对话内容，在空白处填入恰当的句子，使对话完整、通顺。

A：Hi, Alex．You look tired．Why?

B：Because I stayed up late last night．

A：?

B：I had to finish my homework．I'm taking a class online these days．

A：Really!?

B：I'm taking an art class．

A：Is it difficult to learn?

B：．But I have to spend a lot of time doing homework．

A：?

B：I think it's very useful．I can learn a lot about art．

A：That's great．Is the class cheap?

B：Yes，it's cheap，and also helpful． ?

A：Great．I'd love to．