问题如下：
A continuous uniform distribution is sopposed to be U(5, 15). The F(18) is:
选项：
A.50%.
B.80%.
C.100%.
解释：
C is correct.
Because the largest value of the observation is 15, less than 18, so the accumulative probability F(18) should be 100%.
老师，我画的是和您一样的示意图，也理解F(18)求的是F(X≦18）的面积，但是为什么等于100%？？？F（-∞）=0，F（+∞）=1， 难道这个题求的是正无穷？
