At the mould exit of a continuous caster, the metal consisting of a solidified shell with a liquid metal core exits at the rate of 35 kg S^-1. Given that the latent heat of fusion is 3 × 10⁵ J kg^-1 and the total rate of heat removal by the mould is 4.2 × 10⁶ W, the mass fraction of solid at the mould exit is . . . . . . . .
Assume that both while solid and liquid remain at the melting point, they are in the mould.

A. 0.1

B. 0.4

C. 0.8

D. 1.5

Answer: Option B