Consider the following bivariate model: Y = β₀ + β₁X₁ + u.
Which scenario might introduce omitted variable bias into the estimate of β₁?
A. There is no relationship between X₁ and Y.
B. The error term (u) is completely random.
C. There exists a variable X₂ that is correlated with X₁.
D. The model is estimated using experimental data.