水供应
（年龄组：少年组； 难度：中等； 类别：结构）
Beaver has constructed a pipeline system to water his apple tree.
海狸建造了一套管道系统来给他的苹果树浇水。
The expressions contain variables A, B, C, D, which may be true or false. A variable has the value true, if the corresponding gate is open, and false, if it is closed.
这些表达式包含变量 A、B、C、D，它们可能是真也可能为假。如果对应的门是开的，变量的值为真；如果门是关的，则变量的值为假。
In which case the apple tree gets water?
那么苹果树怎么得到水呢？
A: A = false, B = true, C = false, D = false（A 为假，B 为真，C 为假，D 为假）
B: A = true, B = true, C = false, D = false（A 为真，B 为真，C 为假，D 为假）
C: A = true, B = false, C = false, D = true（A 为真，B 为假，C 为假，D 为真）
D: A = false, B = false, C = false, D = trueD（A 为假，B 为假，C 为假，D 为真）
答案：正确答案是 A。
解析：
Since B is open and A is closed, water comes to tree.
正确答案是 A。因为 B 是开着的而 A 是关着的，所以水就流到树那里去了。
B: Since A is open, the water coming from B goes directly down through A.
B：由于 A 是敞开的，从 B 流出的水会直接通过 A 流下去。
C: Since B is closed, there is no water coming from the left source. Since C is closed, the right source is also blocked.
C：由于 B 关闭了，所以没有水从左边的水源流出。由于 C 也关闭了，右边的水源也被堵住了。
D: Since B is closed, there is no water coming from the left source. Since C is closed, the right source is also blocked.
D：由于 B 关闭了，所以没有水从左边的水源流出。由于 C 关闭了，右边的水源也被堵住了。
说明：
Computer programs process data structures that model real things. A model is an abstraction, a simplified image of some real scenario. In this case the gates are represented by variables that contain the values open or closed. This is an abstraction, since all other properties of gates are ignored.
计算机程序处理模拟真实事物的数据结构。模型是一种抽象，是对某些真实场景的简化图像。在这种情况下，门由包含“打开”或“关闭”值的变量来表示。这是一种抽象，因为门的所有其他属性都被忽略了。
Bebras新赛季备赛已开启，扫码领取Bebras真题资料⇓
欢迎咨询【Bebras辅导课程】
