7. In a just-identified IV setup with one endogenous regressor $xi$ and one instrument $zi$, the first stage (with an intercept) is $xi = \pi0 + \pi1 zi + vi$. You estimate $\hat{\pi}1$ with $t$-statistic $t{\pi1} = 1.9$ (two-sided). A common rule-of-thumb uses the first-stage $F$ statistic for $H0: \pi1 = 0$, which in this single-instrument case satisfies $F = t{\pi1}^2$. Which statement is most accurate?
(A) \( F \approx 1.9 \), so the instrument is strong
(B) \( F \approx 3.6 \), suggesting potential weak-instrument concerns and distorted 2SLS inference
(C) \( F \approx 19 \), so 2SLS is more efficient than OLS
(D) \( F \approx 0.5 \), so 2SLS is unbiased in finite samples

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