Which combination of conditions generates omitted variable bias in the following bivariate model?
Y=β₀+β₁X₁+u
a) When there is no relationship between X₁ and Y.
b) When there are no variables in u.
c) When there is a variable X₂ that is correlated with Y.
d) When E(u|X)=0 is true.
e) When there is a variable X₂ that is correlated with X₁.
f) When the model is estimated using experimental data obtained from an RCT.