The reaction that does not proceed in forward direction is

A

$$\text{BeF}_2 + \text{HgI}_2 \rightarrow \text{BeI}_2 + \text{HgF}_2$$

B

$$\text{LiI} + \text{CsF} \rightarrow \text{LiF} + \text{CsI}$$

C

$$\text{CuI}_2 + 2\text{CuF} \rightarrow \text{CuF}_2 + 2\text{CuI}$$

D

$$\text{CaS} + \text{H}_2\text{O} \rightarrow \text{CaO} + \text{H}_2\text{S}$$