In the Following Unbalanced Chemical Equation, What Coefficient Should Be $\rightarrow$

In the following unbalanced chemical equation, what coefficient should be placed in front of H₂O, when balanced?
__PCl₃ + __H₂O $\rightarrow$ __H₃PO₃ + __HCl

A) 2
B) 3
C) 6
D) none