Why Did Robespierre Believe Terror Was a Necessary Measure for the Success

Why did Robespierre believe terror was a necessary measure for the success of the Revolution?

A) He believed it instilled a stronger revolutionary character in the French people, who had lived for too long under a monarchy.
B) He thought that fear was an element that had to be provided by the government, since the Catholic church's authority had been curtailed.
C) He argued that the government should teach or force citizens to become virtuous republicans.
D) He believed that to be more powerful than a monarch, he needed to be more frightening, and so he instituted a regime of terror to maintain authority.