Let \( A \) be a point on the \( x \)-axis. Common tangents are drawn from \( A \) to the curves \( x^{2}+y^{2}=8 \) and \( y^{2}=16x \). If one of these tangents touches the two curves at \( Q \) and \( R \), then \( (QR)^{2} \) is equal to :
\(72\)
\(81\)
\(64\)
\(76\)
