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若直線2ax-by+2=0(a,b∈R)始終平分圓x²+y²+2x-4y+1=0的周長，則ab的取值范圍是____

_________________________.

若直線2ax-by+2=0(a,b∈R)始終平分圓x²+y²+2x-4y+1=0的周長，則ab的取值范圍是(    )

A.（-∞,]                                B.(0,]

C.(0, )                                  D.(-∞, )