How can UT differentiate between lack of fusion and porosity?

• A. Lack of fusion is always larger than porosity.
• B. Porosity cannot be detected by UT.
• C. Lack of fusion usually presents as planar or elongated reflections at the seam with strong spacing; porosity appears as multiple small reflections distributed within the material; use multiple angles and calibration.
• D. Porosity appears as single large reflections.

Answer: (C)