이 문장 뒤에 올 답이 무엇인지 궁금해요

“The application of the theory of the combustion engine to the problem of transportation resulted in the rapid development and the widespread ownership of automobiles, which in turn greatly increased the need for improved road system.”에 이어질 내용을 고르는 문제입니다.
과학 이론을 적용해 오래된 문제를 해결한 결과를 설명하는 것 같은데, 보기 중 어떤 답이 맞을까요?
a. exchanged them for new ones.
b. solved no problems.
c. exchanged them for old needs.
d. solved them at little cost