Look at the graph below, which shows the results of a survey about antibiotic resistance in bacteria.

which question will most likely be investigated based on these results?
a. how long do the bacteria live when they are not exposed to antibiotics?
b. have bacteria always been resistant to antibiotics?
c. is s. aureus weaker than e. faecium?
d. do certain cell structures promote resistance in e. faecium?

i know it's not b, but is it d