题目：
If the number of widgets demanded changes from 51 to 49 when the price changes from $4 to $ 6, the price elasticity of demand is:（）
A. -0.10.
B. -2.00.
C. -1.00.
答案：
参考答案：A
解析：
Price elasticity of demand is calculated by dividing the percent change in quantity demanded by the percent change in price, using the average value of the variable in the computations. The percent change in quantity demanded is (51-49)/[(51+49)/21=0.04. The percent change in price is (4-6)/[(4+6)/2]=-0.40. The price elasticity of demand is 0.04/(-0.4)=-0.10.
