I think the answer is A
Hope this helps have a good night!
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Hazardous substances are dangerous substances while toxins are poisonous substances. Hazardous substances may cause harm to an individuals health; they include acute toxins such as cyanide; substances harmful after repeated or prolonged exposure such as mercury and silica, among others. Toxic substances can be poisonous or cause health effects.
A Thermocline is the temperature of water, while Pycnocline is the density of water. Both are a type of ecoline. I'll update if you need more information.