$12.$ In the manufacture of caustic soda using the membrane cell process, what material is typically used for the membrane?

$-$ a$)$ Cellulose

$-$ b$)$ Polytetrafluoroethylene $($PTFE$)$

$-$ c$)$ Polyvinyl chloride $($PVC$)$

$-$ d$)$ Polyethylene

$13.$ What is the main environmental concern associated with the mercury cell process?

$-$ a$)$ High carbon emissions

$-$ b$)$ Water pollution

$-$ c$)$ Soil contamination

$-$ d$)$ Mercury release