In order to manipulate a variable, there must be at least _____ ...

[B]

In order to manipulate a variable, there must be at least _____ different levels of the variable.
 
  a. one c. three
  b. two d. four

Question 2

In order to experimentally prove that the independent variable is the cause and the dependent variable is the effect we would have to _____
 
  a. observe the dependent variable in the presence of the independent variable
  b. observe the dependent variable in the absence of the independent variable
  c. observe changes in the independent variable when the dependent variable is manipulated
  d. observe changes in the dependent variable when the independent variable is manipulated

[cuttiesgirl16]

Answer to Question 1

B
The requirement is that there be at least two different levels of the independent variable.

Answer to Question 2

D
We must change (manipulate) the cause (independent variable) and observe concomitant changes in the effect (dependent variable).