$21$ The following expression$(s)$ represent periodic signals:

$(a),v(t)=v(t+-k{T}_0),-,v(t)=k{e}^{-3(t+3)},-$

$(e),v(t)=v(t+-k{T}_0),-,v(t)=ku(t+2)e^{-3(t+2)},-$

$(d),v(t)=k{e}^{-3(t+3)},-$

$(e),v(t)=v(t+-k{T}_0),-,v(t)=v(-t),-$

$(c),v(t)=ku(t+2)e^{-3(t+2)},-$

$(d),v(t)=k{e}^{-3(t+3)},-$

$(e),v(t)=v(t+-k{T}_0),-,v(t)=2sin(t_0+-)+2cos(t_n),-$

$(b),v(t)=v(-t),-$

$(c),v(t)=ku(t+2)e^{-3(t+2)},-$

$(d),v(t)=k{e}^{-3(t+3)},-$

$(e),v(t)=v(t+-k{T}_0),-$