In an appropriately designed experiment a scientist is able to test the effect of

The correct answer is an “Independent variable.” Nevertheless, this notion can be interpreted in several manners. Considering that the experiment model is formulated correctly according to the scientific method, the scientist will elaborate a hypothesis which contains a dependent variable, where:

A+B=C

Where A= dependent variable, B= independent variable, and C=predicted (work hypothesis) unpredicted (null hypothesis) result is possible. A dependent variable is where the effect will occur (for example diabetes mellitus) and a Xn independent(s) variable(s) is the one(or those ones) that will modulate it, either in a positive or negative way (a substance-a medication-, an intervention -exercise program- or behavior-change of eating habits-to mention some possible examples) the effect of the independent variable can be controlled in an experimental environment or not, occurring in a spontaneous or random form. Strictly, an experiment happens when both the dependent and the independent variable(s) are entirely controlled. Otherwise, the research is considered a quasi-experimental essay.  

because this was the year that the state of israel was formed