It's high time that we students even harder at our lessons as the national entrance exam

问题补充：

Its high time that we students even harder at our lessons as the national entrance examination is coming nearer.

A. work

B. will work

C. worked

D. have to work

答案：

A. work

B. will work

(答案→)C. worked

D. have to work

解析：It’s high time that +…should do/did+其它“是做…关键时候了”。That从句中要用虚拟语气，特别注意should不可省。故选C.句意：随着高考的临近，是我们学生更加努力学习的关键时候了。