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# If the curves $x^2+py^2=1$ and $qx^2+y^2=1$ are orthogonal to each other, then

$(a)\;p-q=2 \quad (b)\;\frac{1}{p}-\frac{1}{q}=2 \quad (c)\;\frac{1}{p}+\frac{1}{q}=-2 \quad (d)\;\frac{1}{p}+\frac{1}{q}=2$