For three positive integers \( p,q,r \), \( x^{pq^{2}} = y^{qr} = z^{p^{2}r} \) and \( r=pq+1 \) such that \( 3, \; 3\log_{y}x, \; 3\log_{z}y, \; 7\log_{x}z \) are in A.P. with common difference \( \dfrac{1}{2} \). Then \( r-p-q \) is equal to:
\( 12 \)
\( 2 \)
\( -6 \)
\( 6 \)
