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. `(SigmaN_(i)M_(i)^(2))/(SigmaN_(i)M_(i))`
B. `(SigmaN_(i)M_(i))/(SigmaN_(i))`
C. `(SigmaM_(i)^(2)/(SigmaN_(i)`
D. `(SigmaN_(i)M_(i))/(SigmaM_(i)`