$A$ block of mass $m$ is connected to a light spring of force constant $k$. The system is placed inside a damping medium of damping constant $b$. The instantaneous values of displacement,acceleration and energy of the block are $x, a$ and $E$ respectively. The initial amplitude of oscillation is $A$ and $\omega^{\prime}$ is the angular frequency of oscillations. The incorrect expression related to the damped oscillations is

• A
  $x=A e^{-\frac{b}{m}} \cos \left(\omega^{\prime} t+\phi\right)$
• B
  $\omega^{\prime}=\sqrt{\frac{k}{m}-\frac{b^2}{4 m^2}}$
• C
  $E=\frac{1}{2} k A^2 e^{-\frac{b t}{m}}$
• D
  $m \frac{d^2 x}{d t^2}+b \frac{d x}{d t}+k x=0$