Sunflowers growing in pots were placed, with their roots submerged, in the pond contaminated with radioactive elements. The sunflowers kept growing; in the process, they absorbed radioactive elements. Within twelve days, 85 percent of the radioactive elements were removed from the water, which is no less than can be accomplished with the much more expensive conventional filtration techniques. Scientists ttherefore propose using sunflowers for decontamination wherever there are radioactively contaminated ponds. Which of the following, if true, points to a limitation on the applicability of the proposed method of decontamination? ['The water in many ponds contaminated with radioactive elements is so cold that it would kill sunflowers whose roots were submerged in it.', 'Sunflowers that grow with their roots submerged in water grow less well than sunflowers growing under optimal conditions on dry land.', 'Some plants other than sunflowers can also remove radioactive elements from water.', 'In ponds in which the circulation of the water is artificially increased, sunflowers absorb radioactive elements far faster than they do in other ponds.']