Question

The very best atmospheric conditions on Earth result in an angular resolution limit of about 0.4 arcseconds. If an astronomer observes under such conditions at visible wavelengths using a 0.2-meter telescope, the angular resolution is limited by

A. turbulence in the Earth's atmosphere.
B. diffraction effects from the telescope's optics.
C. variable changes in the physical size of the star being observed.
D. diffraction by dust in the interstellar medium.


Please help me. I have no clue..