Which of the following function represents a simple harmonic oscillation 

• A

  $\mathrm{sin\omega t}-\mathrm{cos\omega t}$

• B

  ${\sin }^2\mathrm{\omega t}$

• C

  $\mathrm{sin\omega x}+\sin 2\mathrm{\omega t}$

• D

  $\mathrm{sin\omega x}-\sin 2\mathrm{\omega t}$