Which One of The Following is NOT a Component of Total

Which one of the following is NOT a component of the total variability of measured observations?

[A] Variation between operators
[B] Variability due to operators
[C] Variability between parts dimensions
[D] Variation due to interaction between operators and parts

The correct answer for Which one of the following is NOT a component of the total variability of measured observations is Variation due to interaction between operators and parts