# Which one of the following is most effective in causing the coagulation of an $As_2S_3$ sol?

$\begin {array} {1 1} (1)\;KCl & \quad (2)\;AlCl_3 \\ (3)\;MgSO_4 & \quad (4)\;K_3Fe(CN)_6 \end {array}$

$(2) AlCl_3$