根据材料内容，选择正确答案。

    Two men are walking in the park. One is Jack, a worker. The other is Henry. Jack is going to a chair. It is next to the street. Henry is after him. Jack finds Henry walking after him. He walks faster(更快), because he wants to get to the chair before Henry. Now Henry begins to run because he also wants to get to the chair first. Then he gets to the chair first and sits down on it. When Jack gets to the chair, he sees Henry sitting on it. He is sorry to show a notice（告示）with the words “Wet Paint”（油漆未干）to Henry. Now Henry knows why Jack is going to the chair.

（Ⅰ）证明：AB⊥A₁C；

（Ⅱ）若平面ABC⊥平面AA₁B₁B，AB=CB，求直线A₁C与平面BB₁C₁C所成角的正弦值．

如图所示，内壁光滑长度为4l、横截面积为S的汽缸A、B，A水平、B竖直固定，之间由一段容积可忽略的细管相连，整个装置置于温度27℃、大气压为p₀的环境中，活塞C、D的质量及厚度均忽略不计．原长3l、劲度系数 $k=\frac{3p_{0}S}{l}$ 的轻弹簧，一端连接活塞C、另一端固定在位于汽缸A缸口的O点．开始活塞D距汽缸B的底部3l．后在D上放一质量为 $m=\frac{p_{0}S}{g}$ 的物体．求：