Within six months, nineteen of the forty electric buses in a regional transit fleet suffered sudden battery failures. The failures began shortly after the manufacturer pushed a software update that changes how the batteries are charged overnight. That same season brought a record run of days above 35°C, and many of the affected buses carry battery cells from a single supplier lot. Fleet engineers maintain that the software update is chiefly to blame.

Which of the following, if true, lends the most support to the engineers' attribution?

A. Buses at the fleet's cooler coastal depot, which also received the update, have suffered battery failures about as often as the rest of the fleet.
B. The manufacturer's release notes confirm that the update altered the overnight charging routine.
C. In a chamber held at 38°C, packs drawn from that supplier lot failed repeatedly when running the updated software, while otherwise identical packs running the prior version did not.
D. The buses that failed were overwhelmingly the fleet's oldest vehicles, and battery cells degrade fastest with age.
E. Replacement batteries for the affected buses will not arrive until next spring.