Probability
# Continuous Random Variables

Which of the following CANNOT be the joint probability density function of two continuous random variables $X$ and $Y?$

**(A):** $f(x,y)=1$ for $0\leq x,y\leq1.$

**(B):** $f(x,y)=x+y$ for $0\leq x,y\leq1.$

**(C):** $f(x,y)=xy$ for $0\leq x,y\leq1.$

**(D):** $f(x,y)=e^{x+y}$ for $0\leq x,y\leq\ln2.$