654-2是指
A．东莨菪碱B．山莨菪碱C．阿托品D．后马托品E．普鲁本辛
参考答案：B
设离散型随机变量X的概率分布表为
则E（X4）等于（）．
A.7.2
B.7.9
C.8.3
D.8.5
If a three-digit number is selected at random from the integers 100 to 999, inclusive, what is the probability that the first digit and the last digit of the integer will both be exactly two less than the middle digit ？（）
A. 1:900
B. 7:900
C. 9:1,000
D. 1:100
E. 7:100
则E（X4）等于（）．