题目

College and university graduations in the United States are arriving earlier each year as the average length of school year gets shorter． The school year has fallen by nearly six weeks，from 197days in 1964to 156days in 1994，according to the National Association of Scholars． 1    ，most American kindergarten，elementary（初等的）and high school students are 2    to spend 180days a year in their schools．The length of the average class period also has 3    ，from more than 57minutes to less than 54，the Association reports． “The 4     has to be that students 一定量的某有机物完全燃烧，将燃烧产物通过足量的澄清石灰水，经过滤可得沉淀10 g，但称量滤液时质量只减少2.9 g，则此有机物可能是 [     ]A．C2H5OH    B．C4H8O2     C．C2H6    D．C5H12O