Why is D correct?

Hi Acarrera, thanks for the question! The new detecting method claims to help officials decide "exactly" when to evacuate a town before an earthquake. Useful for this detection is certain changes in the electric current in the earth's crust. Answer choice D, weakens this claim because if there is "considerable variation" between changes in the earth's electric current and the subsequent earthquakes, then how would it help determine "exactly" when to evacuate? It doesn't. It just gives an indication that an earthquake is to come. Hope this helps!