Dalton's billiard ball model was based on which of the following ideas? a. the atom is solid and indivisible. b. a proton is much smaller than a neutron. c. the atom can be spontaneously divided. d. a proton is much smaller than an electron.

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.