Which functions domain consists of all real numbers greater than 3

The picture in the attached figure

we know that

case a) f(x)=3x
the domain is all real numbers------> interval (-∞,∞)

case b) f(x)=√x
the domain is all real numbers greater or equal than 0
the interval [0,∞)

case c) f(x)=3x+4
the domain is all real numbers------> interval (-∞,∞)

case d) f(x)=1/√(x-3)

the denominator can not be zero and can not be negative
so
 (x-3) > 0-----> x > 3-----> the domain is  all real numbers greater than 3
the interval is (3,∞)

the answer is
f(x)=1/√(x-3)

Im pretty sure it would be 6 purple rectangles.