说明：  1、用for和if语句求出供求函数以及稳定时的产量和价格。 2、用多重for循环语句和plot语句画出蛛网模型。 3、制作动画语句。
(1, with for and if statements when you find a stable supply and demand functions and production and prices. 2, with multiple plot statement for loop and draw the cobweb model. 3, animation statements.)