Compare the depend variable to the independent variable.

The independent and dependent variables may be viewed in terms of cause and effect. The difference is that the value of the independent variable is controlled by the experimenter, while the value of the dependent variable only changes in response to the independent variable.

i think that d. wind is the answer.   i missed the question when i answered a. currents.

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.