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

正常纤毛摆动的特点有（）

A.周期性

B.节律性

C.同步性

D.方向性

E.异相性