方框中所给单词的适当形式填空，使对话通顺、正确。每词仅用一次（每空一词）。

Sandy: Mom, I'm really worried.

Mom: Why? What's the ____?

Sandy: I can't find my schoolbag.

Mom: Well, where did you last put it?

Sandy: I can't remember! I ____ a concert yesterday so it might still be in the museum hall.

Mom: Do you have anything ____ in your schoolbag?

Sandy: No, just my books, my pink hair band and some tennis balls.

Mom: So it can't be ____.

Sandy: Oh, wait! I went to a picnic after the concert. I remember I had my schoolbag with me at the picnic.

Mom: So could it still be at the park?

Sandy: Yes. I left early, before the rest of my ____. I'll call them now to check if anybody has it.

如图是和谐号穿越北京居庸关花海的情景，被誉为是开往春天的列车．车内的乘客们欣赏着车窗外的美景，一簇簇鲜花往后“退”去，带给人们“美到窒息”的享受．乘客感到“花海”在运动，是以下列哪个作为参照物？（　　）

​