In the classic 1994 paper, "Does More Intensive Treatment of Acute Myocardial Infarction in the Elderly Reduce Mortality? Analysis using instrumental variables," McClellan, McNeil, and Newhouse, the authors study whether the use of more aggressive procedure, cardiac catheterization, in elderly patients with acute myocardial infarction (AMI) impacts 4-year mortality rates (post AMI).

However, the authors point out that there are important observable and unobservable differences between patients who receive more intensive treatments vs. those who do not. To address this issue, the authors group patients into whether they live relatively close to a hospital that performs these more intensive treatments (such as cardiac catheterization) or relatively far from such hospitals.

The underlying assumption in this setup is that distance to a hospital with the intensive procedure is ___ the type of treatment received and/but ___ the health outcome.

A) not correlated with; does not directly

B) correlated with; directly impacts

C) correlated with; does not directly impact