Match the following Defects to Explanation.

Defects	Explanation
P – Dross	I: Shallow blow found on a flat casting surface.
Q – Scab	II: Lighter impurities appearing on the top surface of casting
R – Scar	III: A rough thin layer of metal, protruding above the casting surface, on top of a thin layer of sand.

The correct option is P-II, Q-III, R-I.

To understand why this match is correct, let us analyze each of the casting defects and their corresponding explanations step-by-step:

1. P – Dross matches with II (Lighter impurities appearing on the top surface of casting):
Dross is a defect in metal casting that consists of metal oxides, slag, and other non-metallic impurities that float to the top of the molten metal. Because these impurities are lighter than the molten metal, they accumulate on the upper surface of the casting during solidification. Thus, P corresponds to II.

2. Q – Scab matches with III (A rough thin layer of metal, protruding above the casting surface, on top of a thin layer of sand):
A scab is a surface defect that occurs when a portion of the mold sand shears or peels off due to the intense heat of the molten metal. When this sand layer flakes off, molten metal flows into the empty space underneath it. The resulting defect appears as a rough, thin projection of metal on the casting surface, lying on top of a thin crust of sand. Thus, Q corresponds to III.

3. R – Scar matches with I (Shallow blow found on a flat casting surface):
A scar is a casting defect characterized by a shallow cavity or depression on a flat surface of the casting. It is caused by the entrapped gases (vapor or air) that cannot escape from the mold cavity during the pouring of molten metal. Thus, R corresponds to I.

Combining these matches, we get:
P – II
Q – III
R – I