Which of the Following Is True About Probability Density Functions

Which of the following is true about probability density functions?

A) A graph of the density function must lie at or below the x-axis.
B) The total area under the density function below the x-axis is 2.0.
C) It calculates the probability of a random variable lying between a certain interval.
D) P (a) x P (b) is the area under the density function between two numbers a and b.