The research mainly deals with the difficulty the students have ________ their attention on their

study all the time. [ ]

A. to focus 

B. to be focused

C. focusing  

D. focused

2013年9月29 13日，中国（上海）           正式启动运行，范围包括上海外高桥保税区、外高桥保税物流园区、洋山保税港区和上海浦东机场综合保税区等4个海关特殊监管区域。

A．关贸总协定

B．自由贸易试验区

C．经济特区

D．特别行政区