In McCabe-Thiele method of theoretical plate calculation for a distillation column, the operating lines of stripping and rectifying sections coincide with the diagonal at ________________ reflux ?

In McCabe-Thiele method of theoretical plate calculation for a distillation column, the operating lines of stripg and rectifying sections coincide with the diagonal at ________________ reflux ?

A. Total
B. Minimum
C. Operating
D. Maximum permissible