A-Level Biology AQA Gas exchange question

I don’t understand how to tackle this question. I’ve tried looking at the mark scheme but have no clue of what’s going on.

Because it's asking about if it's a linear relationship, it wants to know whether the increase in percentage dead cells is the same per increase in concentration. I would find out the increase in percentage of dead cells per 1 ug.cm^-3 increase between 0 and 5, and see if the same increase is done 95 times between 5 and 100. If it's linear it will be the same amount of increase in dead cells per 1 ug.cm^-3 all the way from 0 to 100.