Confounding Often Defeats Attempts to Show That One Variable Causes

Confounding often defeats attempts to show that one variable causes changes in another variable. Confounding means that

A) this was an observational study, so cause and effect conclusions are not possible.
B) the effects of several variables are mixed up, so we cannot say which is causing the response.
C) we don't know which is the response variable and which is the explanatory variable.
D) we would get widely varied results if we repeated the study many times.