D. In practice, we don't know that A regulates D via B and instead want to infer such regulatory relationships. In this example, direct regulation of D by B is only one hypothesis that is consistent with the data - all viable hypotheses are shown by dashed lines. A could directly activate D, C could inhibit D, or A could regulate an unmeasured confounder U which is the true regulator. Direct regulation by a variable Y, which is independent of A, is not possible since the timecourse begins at steady state. 