Why Does the Marketplace of Ideas Often Have a Moderating

Why does the marketplace of ideas often have a moderating effect on people's opinions?

A) Given constant exposure to other people's ideas, it is almost impossible for one not to modify one's own viewpoints.
B) Moderate ideas usually have the best financial backing and the greatest publicity.
C) Those who shop in the marketplace of ideas typically start with moderate viewpoints.
D) Moderate ideas are always closest to the truth and are therefore most successful in the marketplace of ideas.