A first morning sputum is received for culture of mycobacteria. It is digested and concentrated by the N-acetyl-L-cysteine alkali method. Two Lowenstein-Jensen slants are incubated in the dark at 35C with 5-10% CO2. The smears revealed acid-fast bacilli, and after 7 days no growth appears on the slants. The best explanation is:
A) Inadequate incubation temperature
B) Presence of non-viable mycobacteria
C) Insufficient inoculum size
D) Contamination of the culture medium