Scenario Different Species of Bacteria Are Resistant to Different Types of of Antibiotics

Scenario
Different species of bacteria are resistant to different types of antibiotics. For example, one species of bacteria may be resistant to an antibiotic that targets bacterial ribosomes but may not be resistant to a different antibiotic that targets bacterial cell walls. In order to figure out what type of antibiotic(s) a species of bacteria was resistant to, a researcher treated the bacteria with four different antibiotics (A, B, C, and D) and measured how many bacteria were alive over the course of a few weeks. The data that she collected are summarized below.
-Why do bacteria numbers start to decline when treated with antibiotic C but then begin to increase as time goes on?

A) Some bacteria with resistance to antibiotic C were selected for over time.
B) Additional bacteria were added to the experimental group.
C) Antibiotic C degraded and thus was no longer effective.
D) This is the normal course of bacterial growth no matter which antibiotic is applied.