the question

27. When testing a part with a rough surface, it is generally advisable to use : a A (lower frequency search unit and (more) viscous couplant than is used on parts with a smooth surface b. A high frequency search unit and more viscous couplant than is used on parts with a smooth surface c. A high frequency search unit and a less viscous couplant than is used on parts with a smooth surface d. A lower frequency search unit and a less viscous couplant than is used on parts with a smooth surface