Plasma enhancement exponents for carbon fusion reactions in different approximations, neglecting the ion quantum effects, normalized to the enhancement exponent given by Eq. (18), as functions of mass density for 2 isotherms (top panel: T = 10⁸ K; bottom panel: T = 10⁹ K). Dot-dashed lines correspond to the electron rigid background model. Namely, dot-short-dashed lines show h_ii given by Eq. (19) for a rigid background, while dot-long-dashed lines show the approximation of Chugunov & DeWitt (2009b). The other lines correspond to the polarizable electron background case: dotted line: LMR [Eq. (11)]; solid line: h₀ [Eq. (19)]; dashed line: h_appr [Eq. (21)].