Thickness of the medium \[(\mu )\] at which intensity of emergent x-rays becomes half is:

• A. \[\frac{1.23}{\mu }\]
• B. \[\frac{3.318}{\mu }\]
• C. \[\frac{0.126}{\mu }\]
• D. \[\frac{0.693}{\mu }\,\]