东非大裂谷宽约几十至二百公里，深达1000至2000米，是世界上最大的裂谷带，图是东非大裂谷某处的景观，读图，完成下列各题。

完形填空。

     Jean is an English girl. She lives in Beijing 1 her parents. She doesn't know Chinese very 2. But she 3 very hard. She often speaks 4 in class and after class. Sometimes her classmates don't understand her 5 she can't speak Chinese well.

    It's Sunday morning. Jean wants to 6 some flowers, but she doesn't know the 7 to the flower shop. She asks a Chinese boy for help. The boy can't 8 her. Then she takes out a pen and a piece of paper and draws a 9 of a shop and some flowers. The boy looks at it and then 10 her the way.

火车进站时，旅客必须站在安全黄线以内，这样做是因为（     ）