An element with a valence of 8 are most likely noble gases (He, Ne, Ar, etc) which means they're already stable so they dont need to undergo chemical bonds with another atom because they already have a full valence shell.
A or B... I mean when lava is poured out to the surface, heat and soilica content may play a role... That's what I think sorry :(
I found the Hydrolic system, im not sire if its helpful or not. I could only find that one, sorry if it doesn't help but I hope it does ^-^
$0.60
90 mins is 1.5 hrs
5 kW × 1.5 = 7.5 kWh
7.5 kWh × $0.08 = $0.60