题目

How can a creature weighing over 5 tons and normally taking 150 kilograms of food and 120 liters of water per day survive in a desert environment? In the southwest African country of Namibia, and the Sahara lands of Mali further north, the desert elephant does just that. Although not regarded as a separate species from the African elephant, the desert cousin differs in many ways. Their bodies are smaller, to absorb less heat, and their feet are larger for easier walking across sandy surfaces, They are taller, to reach higher branches. They have shorter tusks（象牙）, and most importantly, 当k= 时，多项式x-2kxy+3y-5+4xy中不含二次项．