An Astronomer Observing the Spectrum of the Sun on the Solar

Multiple Choice

An astronomer observing the spectrum of the Sun on the solar equator finds that the Balmer H _$\beta$ spectral line ( $\lambda$ = 486 nm) is blueshifted by 0.0033 nm when measured at one edge of the Sun's disk compared with the same line at the center of the Sun's disk and is redshifted by the same amount on the equator at the other side of the Sun's disk. If this Doppler shift is due to the Sun rotating (try drawing a diagram) , then the rotational speed of the Sun at its equator is

A) 1 km/s.
B) 0.5 km/s.
C) 2 km/s.
D) 4 km/s.