In a quality control (QC) procedure on a new batch of Mueller-Hinton plates using a standard QC stock strain of Staphylococcus aureus, the disk inhibition zone sizes for three of the drugs tested were...

In a quality control (QC) procedure on a new batch of Mueller-Hinton plates using a standard QC stock strain of Staphylococcus aureus, the disk inhibition zone sizes for three of the drugs tested were too small and fell below the expected QC range. Which of the following is the most likely reason for this observation?

a. These three antibiotic disks were outdated and had lost potency b. These three disks were faulty in that the antibiotic content was too high c. Bacterial suspension of Staphylococcus was probably contaminated with another organism d. The plates received insufficient incubation time