下圖結構中\mathrm{R}_{1}與\mathrm{R}_{2}為何種取代基時，則為pyrimethamine？
\mathrm{R}_{1}
\mathrm{R}_{2}
\mathrm{R}_{1}＝H，\mathrm{R}_{2}＝\mathrm{CH}_{3}
\mathrm{CH}_{3}
\mathrm{R}_{1}＝F，\mathrm{R}_{2}＝\mathrm{C}_{2}\mathrm{H}_{5}
\mathrm{C}_{2}
\mathrm{H}_{5}
\mathrm{R}_{1}＝Cl，\mathrm{R}_{2}＝\mathrm{C}_{2}\mathrm{H}_{5}
\mathrm{R}_{1}＝\mathrm{CH}_{3}，\mathrm{R}_{2}＝F
