What Most Clearly Distinguishes Experiments from Other Types of Research

Multiple Choice

What most clearly distinguishes experiments from other types of research is that:

A) experiments involve controlled observation and the manipulation of the value of variables.
B) experiments are based on asking people to answer questions about their beliefs and their behavior.
C) experiments are conducted in artificial settings where aspects of the natural environment are simulated.
D) experiments assess the effect of independent variables on dependent variables.