B
I would be willing to bet my testicle that your 2nd battery was not dead. The isolator will isolate when the voltage drops below it's set threshold. What you experienced is the isolator would not close back together because the voltage was too low. Did you actually check your second battery and see if it was dead? If you would have jumpered over the isolator, I would imagine your machine would have fired up.
EDIT** I have lost this testicle atleast 7 times in the past.
EDIT** I have lost this testicle atleast 7 times in the past.