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The number to the right of an element's symbol (ex. c-12) identifies the of an isotope.

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.

Draw the aromatic compound toluene (methylbenzene). show all hydrogen atoms, including those on the ring.