【问】
The China Railway Corporation ____ (busy) itself in laying down over twenty-five thousand kilometers of high-speed tracks over the past ten years.
答案：has busied或has been busying
问题：本题的思路是什么？
【答】
本句的busy用作动词，意为“使…忙”，因与句末的over the past 10 years搭配，所以用现在完成时。由上可知，答对本题，需要明确两点：
一、busy不是形容词而是动词，这种形容词与动词兼类的情况并不罕见，如wrong和calm：
- She felt deeply wronged by his accusations. 他的指控使她觉得深受冤枉。（剑桥词典）
- He tried to calm the screaming baby by rocking it back and forth. 他来回摇晃着嚎哭的婴儿，想让他安静下来。（剑桥词典）