17.若已知某位置的假體劑量為Dmed，對光子射束而言，相對應等效位置的水中劑量Dwater為何？（：mean mass energy absorption coefficient，：average restricted mass collisional stopping power of electron，：average mass stopping power，：mass attenuation coefficient） (A)Dwater = Dmed × (μen/ρ) water / (μen/ρ) med (B)Dwater = Dmed × (L/ρ) water / (L/ρ) med (C)Dwater = Dmed × (μ/ρ) water / (μ/ρ) med (D)Dwater = Dmed × (S/ρ) water / (S/ρ) med