The Most Likely Origin of the "Stab in the Back

The most likely origin of the "stab in the back" theory associated with World War I was

A) genuine suspicion among the German population, which had been kept in the dark about Germany's faltering condition.
B) the bitterness of the tsar's supporters after the February (March) Revolution.
C) a growing awareness among citizens of France and Britain that their governments did not have their best interests in mind when they went to war.
D) Italy's failure to aid the other members of the Triple Alliance.
E) based on the alarm and surprise that Germany had actually been invaded, although the public felt victory was near.