日常生活中，对刚是怎样理解的？
参考答案：硬，强，直，重，定。宁断不弯，都会被理解为刚。
It's 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
栈的基本运算有三种：入栈、退栈和 【1】 。
