题目

       Despite the negative image of nuclear energy, actually its public safety risks are no greater than ______ of fossil fuels.     A. one             B. that             C. them            D. those 答案：D15．若0＜x1＜x2，0＜y1＜y2，且x1+x2=y1+y2=1，则下列代数式中值最大的是（　　）A．x1y1+x2y2B．x1x2+y1y2C．x1y2+x2y1D．$\frac{1}{2}$