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

淘汰值是在考虑种子生产者利益和有较少失误的基础上，将样本内观察到的变异株数与规定的标准比较，作出接受或淘汰种子批决定的数值。请指出一般在哪种情况下可以利用淘汰值，并简要说明理由。