单选题 Einstein's theory of relativity seemed hard to believe at the time ______.
A. when he first introduced B. that he first introduced it
C. he first introduced D. which he first introduced it

【正确答案】 B

【答案解析】[解析] 定语从句用法
at the time后接定语从句可以用when和that引导。故排除C项和D项。A项成分残缺，故排除。答案为B项。