By Paul Monies Business Writer pmonies@oklahoman.comOklahoma's gross production tax collections hit their lowest level in two decades last year, driven down by combination of lower oil and natural gas prices and lower effective tax rates. The sharp decline has revived the debate over the optimum rates of taxation for pulling oil and natural gas out of the ground in Oklahoma amid a new era of production driven by horizontal drilling and hydraulic fracturing.Read more on NewsOK.com