One of the Earliest Unions-If It Could Be Called That-Were

One of the earliest unions-if it could be called that-were stonemasons. During Medieval Europe, the skill of masonry was a closely guarded secret and one had to endure a seven-year apprenticeship before being allowed to independently practice the trade. On the completion of the apprenticeship, the new mason would receive his own set of tools (a critical form of compensation since the tools were expensive and securing a loan was very difficult) and join the union of masons (allowing the new mason to charge a high price for his labor) . As a result, hiring masons was very expensive. What made this union particularly successful?

A) It was time consuming to learn the secrets of masonry.
B) It was costly to buy the physical tools of masonry.
C) It was difficult to enter the industry without the union's approval.
D) It was hard to make the physical tools of masonry.