The levels of mercury in two different bodies of water are rising. In one body of water the initial measure of mercury is 0.05 parts per billion (ppb) and is rising at a rate of 0.1 ppb each year. In the second body of water the initial measure is 0.12 ppb and the rate of increase is 0.06 ppb each year.

For each body of water, the mercury level after y years is its initial level plus its yearly increase multiplied by y. The first level is 0.05 + 0.1y, and the second is 0.12 + 0.06y. Setting these levels equal gives 0.05 + 0.1y = 0.12 + 0.06y, which is choice C.

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