Coal and wood are two sources which had been used extensively since the 18th century. when steel was commercialized heavily using the Bessemer Proces later at the 19th century. At this time, petroleum became the major energy source of energy and was even used extensilvely in other processes as converting petroleum to fuels.
By the end of the 13th century, Crusading had become more expensive. The time had passed when a Crusade army was made up of knights who served under a lord and paid their own way. Economic pressures caused many nobles to seek royal service. Royal armies, therefore, became more professional, and many knights as well as foot soldiers served for pay. Moreover, the rise of royal authority meant that great Crusades could no longer be cobbled together by feudal lords but were increasingly reliant on kings, who were by their nature easily distracted by events at home.
The answer to this question is C
