SAT Boundaries Question

As the exoplanet 81 Cetib orbits a star 330 light-years from Earth, the exoplanet's gravity causes the star to wobble. In 2008, astronomers this wobble through shifts in the color of the star's spectral light — blueshifts indicating longer wavelengths and movement toward the observer, redshifts shorter wavelengths and movement away — deduced that the fluctuation was caused by the gravitational force of a previously undetected exoplanet. Which choice completes the text so that it conforms to the conventions of Standard English?