These books were very dear to him and he bought them at ______ expense when he was studying abroad.

【正确答案】 C

【答案解析】 根据句中的“dear”可知，这些书花费很大。“considerable”意为“相当大的”，符合题意。“considerate”“体贴的，考虑周到的”；“considered”“经过仔细考虑的”；“considering”“考虑到，鉴于”，均不符合。故答案为C。