中东地区一些国家的人们夜晚常把床铺安排在屋顶上，这是因为（    ）

1929—1933年，资本主义世界发生了一空前严重的经济危机，使美国的经济政策由胡佛的自由放任转变为罗斯福的国家干预。你认为这一变化的实质是（    ）

在“探究光对鼠妇生活的影响”实验中，将实验盒一边遮光，另一边不遮光，这是为了 

    These are Cara and Ben. They are twins (双胞胎). They are twelve. This is their bedroom. It's a nice room. There are two beds in the room. The yellow bed is Cara's and that green one is Ben's. The twins have one desk and two chairs. The chairs look the same. Cara's sweater is on his bed. Ben's coat is on his chair. Their clock, books and pencil-boxes are on the desk. Their schoolbags are behind the chairs.