What Was the Fatal Calculation of the Northern Irish Government

What was the fatal calculation of the Northern Irish government?

A) They had anticipated that most of their citizens were Irish Catholic and wanted to be free from the British rule, but most of them were actually fine with it.
B) They expected that the Black and Tan War would only last a few months, but as the conflict dragged on over a year, the will to win slowly faded.
C) The government had expected that if they helped the British keep control of the rest of Ireland, they would govern Ireland (under the British observation) but instead all of Ireland was subservient to Britain.
D) They used the RUC and a hated reserve force known as the B-Specials to crack down on civil rights workers and demonstrators.