题目

Although men and women have been growing crops for thousands of years, they have not always had enough to eat. Even today, many people in the world go hungry, and even die of hunger. If we are to grow enough for everybody, we have to grow more crops and better crops.Plants need the right food in order to grow. In particular, they need three chemical substances (物质) in the soil: nitrogen (氮), phosphorus (磷) and potassium (钾). They also need smaller amounts of other chemical substances like calcium. If there is too little of any of these substances in the soil, plants can’t grow well已知为正实数，且，若对于满足条件的恒成立，则的取值范围为（ ）（A） （B）（C） （D）