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.

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.

wavelegth as in spectrum red oygbiv