题目

 (08·福建D篇) The global energy crisis is approaching.What can we do?Here are some steps you can take. Cooling puts the greatest stress on your summer energy bill and the power grid（电网）.Just as a tune-up for your car can improve your gas mileage,a yearly tune-up of your heating and cooling system can improve efficiency and comfort.Clean or replace filters monthly or as needed. For central air conditioning systems and room air conditioners,look for the ENERGY STAR,the federal government’s symbol for energy efficiency.For central air,purchase the system with the highest p化合物XY和ZY2中Y的质量分数分别为40%与50%，则化合物XZY4中X的质量分数为（　　） A、20%B、30%C、40%D、50%