If`N_(1),N_(2),N_(3),……`are the number of molcules with molecular masses `M_(1),M_(2),(M_(3),…`respectively ,then mass average molar mass is expressed as:
A. `(SigmaNiMi^(2))/(SigmaNiMi)`
B. `(SigmaNiMi)/(SigmaNi)`
C. `(SigmaMi^(2))/(SigmaNi)`
D. `(SigmaNiMi)/(SigmaMi)`