What is the best strategy for investigating why the fish in your fish tank keep dying?
a. dissect the dead fish and determine a cause of death.
b. observe the behavior of the fish over the period of a day.
c. experiment with one fish tank variable at a time, like water temperature or ph.
d. conduct a literature review of the best environmental conditions for tropical fish. eliminate

c.experiment with one fish tank variable at a time, like water temperature or pH.  

An independent variable in an experimental study which can be changed or manipulated in an experiment to observe it's impact on the dependent variable. A dependent variable is the variable which cannot be changed in an experiment by human effort. It is the final outcome of the experiment.

In the given situation, to investigate the fact that why the fish in the tank keep drying an experiment with one fish tank variable at a time, like water, temperature or pH is a best strategy. The temperature, time, and pH are independent variables, the effect of which should be observed on the dying fishes which are dependent variable.

a displacement distance is size of displ