阅读理解，选择正确的答案。

    April Fools'Day is a day to play jokes on others．No one knows how it began，but people think it first began in France．

    Each country celebrates April Fools'Day differently．In France，people call the April fools "April Fish"．They tape a paper fish to their friends' backs to fool them．When he or she finds this，they shout” April Fish!”

    In the UK，people play jokes only in the morning．You are a“noodle”if someone fools you．

    In the US，people play small jokes on their friends on the first of April．They may point down to your shoe and say，“Your shoelace iS untied．”If you believe them and look down，you are an April fool then．

已知集合M={（x，y）|y=f（x）}，若对于任意（x₁，y₁）∈M，存在（x₂，y₂）∈M，使得x₁x₂+y₁y₂=0成立，则称集合M是“Ω集合”．给出下列4个集合：

①M={（x，y）|y=$\frac{1}{x}$}

②M={（x，y）|y=e^x﹣2}

③M={（x，y）|y=cosx}

④M={（x，y）|y=lnx}

其中所有“Ω集合”的序号是（　　）

确定一个圆有两要素，一是____ ，二是____ ．

判断下面的词语是否正确。

花篮