Redshift distribution as a function of GRB brightness at the time of observation (middle panel) and dust extinction, A_V (bottom panel). Blue filled circles denote the GRBs in our statistical sample with C I detected in absorption. Black empty circles mark the GRBs in our sample without C I, where triangles represent upper limits on A_V. In the middle and bottom panels, the dashed lines show the mean acquisition magnitudes and A_V values for the GRBs without (black) and with (blue) C I detected in absorption in different bins: 1 <  z <  2, 2 <  z <  3, 3 <  z <  4, and z >  4. In the top panel the fraction of GRB C I absorbers is shown in the same redshift interval. The GRBs with C I detected below the completeness limit are shown by the blue empty circles, and are not included in the analysis.