单选题
After two years of working on the subject, he seriously switched his field again, ______ to comparative anatomy and physiology. A. transferring B. gliding C. transforming D. drifting
【正确答案】
A
【答案解析】选择答案A的参照是短语switched the filed。总体上说，选择正确答案的一个重要途径是寻找参照。正可谓：只有比较，才有鉴别。