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．
