I have written the expression for a char variable x.The isupper(x) will return true if the character x is upper case and false if the character x is lower case.
I have stored the returned value to a bool variable b .So the value of variable b will be true only when the x is in uppercase and false when b is lower case.
This is complicated because the scales on the x-axis and y-axis are not the same. Graph D has the correct y-intercepts and the correct slopes. The solution is x = 500, where the two lines intersect.