—I studied in the United States for 3 years. —.A. So I did.B. So was I.C. So did I.D. So d

问题补充：

—I studied in the United States for 3 years.

—.

A. So I did.

B. So was I.

C. So did I.

D. So do I.

答案：

A. So I did.

B. So was I.

(答案→)C. So did I.

D. So do I.

解析：考查倒装句：句意：--我在美国学习了三年。--我也是。So+be动词/情态动词/助动词+主语，意思是“某人也是”，因为第一句话是实意动词studied，所以答语是助动词did，选C。