A first morning sputum is received for culture of acid-fast bacilli. It is digested and concentrated by the N-acetyl-L-cysteine alkali method. Two Sabouraud dextrose slants are incubated in the dark at 35ºC with 5%-10% C0₂. The smear reveal acid-fast bacilli, but the slants show no growth after 8 weeks. The explanation is:
a) improper media used
b) incorrect concentration procedure used
c) improper specimen submitted
d) exposure to C0₂ prevents growth