参考答案

正确答案：C
解析：concentrate的意思是“专心，集中”，例如：I tried to read a few pages，but I found it hard to concentrate．我试图看几页，可是很难集中精力。base的动词意思是“以……作基础”，例如：Their relationship was based upon mutual respect．他们的关系是以相互尊重为基础的。focus的动词意思是“(使)聚焦，集中”，例如：He stared out the window for a moment，trying to focus his thoughts．他朝窗外凝视了一会儿，试图使自己集中精力。