When prices are low，people will buy more，and when prices are high，they will buy less．Every shopper knows this．But at the same time，producers want higher prices for their goods when they make more goods．According to the economic（经济的）point of view，changes in the prices of goods cause changes in supply and demand． As is shown in the graph（图表），people buy fewer shoes as the price of shoes goes up．On the contrary，a decrease in the price causes an increase in demand．

Business firms look for the perfect price at which the largest profits（利润）can be made．If the price of the shoes goes up to 50，the consumers（消费者）will not buy all of 3，000 shoes．The producers will have a surplus（剩余）of 2，000 shoes and they can only get 50，000．If the price of the shoes is lowered to10，as many as 5，000 shoes can be sold．Still，only 50，000 is made．
1．According to this article，the main idea of the first paragraph is that ____ ．
A．the lower the price is，the less people will buy
B．producers want to make more money by making fewer goods
C．every shopkeeper knows the price for goods
D．how changes in prices of shoes affect the number of shoes sold
2．From the passage，we can conclude that ____ is the best price for the producers to sell the shoes which are worth the highest price．
A.30       B.20        C.10          D.50