During the dialogue with Chinese youth in Shanghai President Obama announced the U.S. wou

问题补充：

During the dialogue with Chinese youth in Shanghai, President Obama announced the U.S. would expand the number of students to study in China to 100,000 ties between the two countries.

A. strengthening

B. strengthened

C. to strengthen

D. to be strengthened

答案：

A. strengthening

B. strengthened

(答案→)C. to strengthen

D. to be strengthened

解析：表示做某事的目的用动词不定式to do，strengthen为加强的意思，故本题选C。