R: CO2 Level Over Millenia Project: hline() label

Hi All,
I am working on CO2 Level Over Millenia Project. At the last step (18), there is requirement to put a label to the horizontal line which represents the Historical CO2 Peak before 1950. The hint is Assign the value of the linetype argument as "Historical CO2 Peak before 1950"
However, my understanding of linetype is the visual type of the line, for example, solid, dashed, etc.
Which argument should I use to label the line?