All selected candidates plotted in different planes. Top left panel: i-W3 distribution of candidates selected by different methods. Top right panel: Candidates selected by different methods plotted in 2 − 10 keV to optical flux ratio vs. r-W1 colour plane. Bottom left panel: 0.2 − 2.3 keV to optical flux ratio vs. r-W1 colour. The sources from different selection methods are indicated with different colours or shapes as shown in the legend. Bottom right panel: All selected candidates plotted in i-W4 vs. r-W1. The sources from different selection methods are indicated with different colours as shown in the legend.