the application of the theory of the combustion engine(연소 기관) to the problem of transportation restulted in the rapid development and the widespread ownership of automobiles, which in turn greatly increased the need for improved road system. thus the application of a scientific theory in solving old problems (뒤에 이어지는 말 선택하기)

a. exchanged them for new ones.
b. solved no problems.
c. exchaned them for old needs.
d. solved them at little cost

답을 a라고 해야할지 b라고 해야 할지 .....
좋은 답변 부탁드리겠습니다.