Which of the following statements is true about the composition ( f circ g ) of two linear functions ( f(x) = ax + b ) and ( g(x) = cx + d ) with ( a ), ( b ), ( c ), ( d ) as constants?
a) ( f circ g ) is a quadratic function
b) ( f circ g ) is a cubic function
c) ( f circ g ) is a linear function
d) ( f circ g ) is an exponential function