W-Ni compact is prepared by liquid phase sintering is 1500°C. If the size of the tungsten grains is 40µm and the interfacial tungsten- tungsten and tungsten-nickel energies are 0.52 and 0.30 Jm^-2 respectively, the  predicted average neck size (in µm) of sintered tungsten grain is:

(Melting points of tungsten and nickel are 3410°C and 1453°C, respectively)

(A)   10   (B)  15   (C) 20   (D) 25