Assume that a random variable can be characterized with the density: ( f(x) = begincases 0, & if x < 0 x, & if 0 ≤ x ≤ 2c endcases ) What is the value that ( f(c) ) would need to take on so that this would be a valid density function? 
A) 1 
B) 2 
C) 0.5 
D) 4