A level biology question gas exchange

Can someone help me with this question i looked at the mark scheme and it still does not make sense i don't know which calculations i should make.

Linear means a constant gradient:
So the change in Y (% dead cell)/ change in X (concentration) would be the same between (0 and 5 conc), (5 and 100 conc) if it was linear.
for 0 to 5: (15-6)/5 = 9/5
For 5 to 100: (42-15)/95 = 27/95
As 27/95 does not equal 9/5, there is no linear relationship.
Alternatively could have used (0 and 100 conc).