如图，点A、C、B在⊙O上，已知∠AOB =∠ACB =α．则α的值为（    ）

材料二、“以无产阶级革命军队推翻资产阶级，由劳动阶级重建国家，直到消灭阶级差别”。“采用无产阶级专政，以达到阶级斗争的目的-----消灭阶级。”“资本私有，没收一切生产资料，如机器、土地、厂房、半成品等，归社会所有。”

-----中共“一大”通过的第一个纲领

请你回答：

   My name is Chen Lan. My home is in Gulangyu. Do you know it? It is in Xiamen. It is near the sea(海). Gulangyu is a small place，but it is very nice and clean. There are no cars，no buses. People only walk. So it is very quiet.
   Our house is in the middle of Gulangyu. Behind our house there is a big old tree. My grandfather tells me that the tree is very，very old. There are many birds in the tree. We call it a “bird tree”. Our house is near the sea. The sea is big and blue. There are a lot of fish in the sea. After school，I go there and catch(捉) fish with my friends. It is very interesting. I like fish and I like catching fish.

液体在水中存在____现象，液体在水中的沉浮与____无关。