Suppose that the modulating signal is m m(t)=2 \cos \left(2 \pi f_{m} t\right) and the carrier is x_{C}(t)=A_{C} \cos \left(2 \pi f_{c} t\right) . Which one of the following is a conventional AM signal without over-modulation?

(a) x(t)=A_{c} m(t) \cos \left(2 \pi f_{c} t\right)

(b) x(t)=A_{c}[1+m(t)] \cos \left(2 \pi f_{c} t\right)

(c) x(t)=A_{C} \cos \left(2 \pi f_{c} t\right)+\frac{A_{C}}{4} m(t) \cos \left(2 \pi f_{c} t\right)

(d) \begin{aligned}x(t) &=A_{C} \cos \left(2 \pi f_{m} t\right) \cos \left(2 \pi f_{c} t\right) \\&+A_{C} \sin \left(2 \pi f_{m} t\right) \sin \left(2 \pi f_{c} t\right)\end{aligned}