How can you tell if a solution is concentrated

What mass of natural gas (ch4) must you burn to emit 276 kj of heat?

15. which of the following is not human-caused groundwater pollution? a. water in an aquifer dissolves elements such as arsenic and mercury from surrounding rock. b. water in an aquifer is contaminated by leachate that seeps into the ground from a landfill. c. water in an aquifer becomes polluted with chemicals used in hydraulic fracturing, or fracking. d. water in an aquifer absorbs harmful bacteria from the drainage field of a septic tank.

Reactions in cells take place at about a. 40°c b. 0° c. 100°c d. 60°c