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.

哈维是英国生理学家，曾做如下实验：用丝带扎紧人的上臂时，发现丝带下方的静脉隆起而动脉却变得扁平；在丝带上方的动脉隆起而静脉扁平．说明了血流方向（    ）