[单项选择]Science is an enterprise concerned with gaining information about causality, or the relationship between cause and effect. A simple example of a cause is the movement of a paddle as it strikes a ping-pong ball; the effect is the movement of the ball through the air. In psychology and other sciences, the word "cause" is often replaced by the term "independent variable". This term implies that the experimenter is often "free" to vary the independent variable as he or she desires (for example, the experimenter can control the speed of the paddle as it strikes the ball). The term "dependent variable" replaces the word "effect", and this term is used because the effect depends on some characteristic of the independent variable (the flight of the ball depends on the speed of the paddle). The conventions of science demand that both the independent and dependent variables be observable events, as is the case in the ping-pong example. In the case of biorhythm theory, the independent variable is
A. cause and effect can be independent of each other.
B. there is hardly anything that happens without a cause.
C. dependent and independent variables affect each other.
D. cause and effect may vary respectively in most events.