Identify the Implied Population in the Information Below

Identify the implied population in the information below. Government agencies carefully monitor water quality and its effect on wetlands (Reference: Environment Protection Agency Wetland Report EPA 832-R-93-005) . Of particular concern is the concentration of nitrogen in water draining from fertilized lands. Too much nitrogen can kill fish and wildlife. Twenty-eight samples of water were taken at random from a lake. The nitrogen concentration (milligrams of nitrogen per liter of water) was determined for each sample. The variable in this information is nitrogen concentration (mg nitrogen/l water) .

A) number of fish
B) samples of water taken at random
C) none of these choices
D) nitrogen concentration (mg nitrogen/l water) in the entire lake