Test A: sensitivity of the C¹⁷O/¹³C¹⁸O(2–1) ratio to independent variations in isotopic abundances. Top left: absolute ratio as a function of C¹⁷O(2–1) optical depth when varying ¹²C/¹³C by ±30% at fixed ¹⁸O/¹⁷O. Top right: same, but varying ¹⁸O/¹⁷O at fixed ¹²C/¹³C. Bottom: all curves normalized by their optically thin limits. The nearly identical shapes confirm that the ratio-τ response is insensitive to these variations.