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Answer:
I believe the answer is D
The conclusion that coral does not grow well in conditions with high salt is invalid because (B) The experiment changes more than one variable at once.
In order to correctly study the effect of three variables on there needs to be, 3 x 3 x 3 = 27 different aquariums (every variable takes 3 different values and there are 3 different variables).
In these aquariums, only one condition needs to be varied at a time to study its effect. To study the condition with high salt, the temperature and pH needs needs to be kept fixed at a particular value whilst varying the salinity from normal to low to high.
