In an experiment, 3 different bacterial samples a,b and c were treated with an antibiotic. The following graph
shows the number of surviving cells (Y axis) in the three bacterial samples after exposure to the antibiotic for indicated time (X axis). What could explain the differences in slopes of lines a, b and c (in other words, how is the antibiotic affecting the survival of the three samples)? In case of sample a, what could be causing this kind of response to the antibiotic?