Which of the following statements is true?

A.	A histogram is created by drawing rectangles whose bases correspond to the class intervals, and the area of each rectangle equals the total number of observations in the sample.
B.	A histogram can be used for only categorical variables.
C.	A histogram is created by drawing rectangles whose bases correspond to the number of observations in each class.
D.	A histogram is created by drawing rectangles whose bases correspond to the class intervals, and the area of each rectangle equals the number of observations in that class.