Austin had made no grammar mistakes in his thesis paper, but ______ had he well prepared for it.
【正确答案】 B
【答案解析】but连接前后两个分句，形成转折关系，表明后面分句应是否定意思，且空格后是部分倒装句式，可见句首应填入含有否定意义的词，答案为B。