Most people have flown a kite or have seen one ride and dip(下降）in strong spring wind. Not so many people know that kites were first made in China thousands of years ago. The ancient Chinese were making and flying kites even before they were writing.

    A long time ago, the Chinese made kites to use in wars. They would fly these war kites in the dark. The kites were fixed(固定）so that they made strange sounds. Men at war would hear these sounds and run away. They thought those strange sounds were made by gods in the sky.

    The ancient Chinese also flew kites to bring good luck and to make their crops grow rich and tall. Sometimes they tied long strings(细绳）and hooks(钩) to their kites. Then they would fly the kites over water, letting the hooks hang down to catch fish.

    The Chinese use sticks(树枝), strings and paper for their kites. Some of these kites look like animals or trees. Others look like birds or houses.

下列各句中，没有语病的一句是（   ）

①若对于任意x∈R都有ff（x）=f（x）成立，则f（x）=x．

②若对于任意x∈R都有ff（x）=x成立，则f（x）=x．

③若存在唯一的实数a，使得fg（a）=a成立，且对于任意x∈R都有gf（x）=x²﹣x+1成立，则存在唯一实数x₀，使得g（ax₀）=1，f（x₀）=a．

④若存在实数x₀，y₀，fg（x₀）=x₀，且g（x₀）=g（y₀），则x₀=y₀．

其中是真命题的序号是____．（写出所有满足条件的命题序号）

2015年***政府工作报告指出，2014年建立了统一城乡居民基本养老保险制度，企业退休人员基本养老金水平又提10%。基本建成保障性安居工程511万套。政府此举（   ）