已知点$M\left(a,b\right)\left(a>0,b>0\right)$是圆$C:x^2+y^2=1$内任意一点，点$P\left(x,y\right)$是圆上任意一点，则实数$ax+by-1$（   ）

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                                                                                                       春兴（唐）武元衡
                                                                                                杨柳阴阴细雨晴，残花落尽见流莺。
                                                                                                春风一夜吹乡梦，又逐春风到洛城。