What is the "omitted variable" problem in determining cause and effect?

A) It is a problem that arises when an economic variable that affects other variables is omitted from an analysis and its omission leads to false conclusions about cause and effect.
B) It is a problem that arises when a significant variable is not given enough weight in an economic experiment leading to skewed conclusions about cause and effect.
C) It is a problem that arises when an insignificant economic variable that should have been omitted is included in an economic experiment leading to false conclusions about cause and effect.
D) It is a problem that arises when an insignificant variable is given too much weight in an economic analysis leading to skewed conclusions about cause and effect.

A