Mycobacterium tuberculosis is called acid-fast because:

<p><strong><span>a) It is easily stained by gram stain</span></strong></p> <p><strong><span><span>b) Once stained, it can not be remove by acid</span></span></strong></p> <p><strong><span>c) It is easily decolorizes after staining.</span></strong></p> <p><strong><span>d) It is stained by an alkaline stain</span></strong></p> <p><strong><span>e) None of the above</span></strong></p> <p> </p> <p><span><strong>f) All of the above</strong></span></p>