问题
单项选择题
The cost C of manufacturing a certain product can be estimated by the formula C=0, 03rst2, where r and 8 are the amounts, in pounds, of the two major ingredients and t is the production time in hours. If r is increased by 50 percent, s is increased by 20 percent，and t is decreased by 30 percent, by approximately what percent will the estimated cost of manufacturing the product change ？（）
A．40%increase
B．12%increase
C．4%increase
D．12%decrease
E．24%decrease
答案
参考答案：D
解析：
变化后的r，s和t分别为1.5r，1.2s，0.7t，根据题意可得该产品的估计成本变化的百分比为：[*]11.8%又因为1.5×1.2×0.72＜1，所以成本将减少大约12%。