阅读下列材料：
“美利坚合众国政府承认中国的立场，即只有一个中国，台湾是中国的一部分。”
——摘自中美建交公报
请回答：
（1）中美建交是在哪一年？
（2）中美建交对于双方和世界来说有哪些积极作用？

阅读理解

    Many other people and I in Britain love charity shops because we can find them on every street．The charity shops sell all kinds of things and they are very cheap．

    The first charity shop was opened by Oxfam in 1947．Now there are over 7,000 charity shops in the UK．My favorite charity shop in my hometown is the Cross Shop．There I always find children’s books，all 10 to 20 pence each．They are really cheap．

    Most of the shop workers in charity shops are volunteers，    each shop has a manager and he gets some money．Every morning you see bags of things outside the shops．Some people bring and put them there without waiting for thanks．In fact，over90％of the things in charity shops are from kind people．

    All the money the shops get goes to charity work．Charity shops raise more than 110 million pounds every year．The money is for sick and poor children。homeless and disabled people，and many others．In a charity shop you can get cheap but nice things．You might even feel special while shopping．

    If possible，let’s do something for charity together．

根据短文内容，完成下列各题。

将2mol SO₂和2mol SO₃气体混合于固定容积的容器内,在一定条件下,发生反应:2SO₂ + O₂ 2SO₃,达平衡时SO₃为n mol。相同温度下分别按下列配比,平衡后SO₃的物质的量大于n mol的是（    ）