单选题 The tendency of the human body to reject foreign matter is the main obstacle to successful organ transplantation.
A. factor B. constituent C. barrier D. break

【正确答案】 C
【答案解析】[解析] 题干中的obstacle意为“妨碍”。A项“要素”；B项“成分”；C项“障碍”；D项“裂口”，只有C项与题干种划线单词意义相近。