Geology

Step 1: Recall the hardness values of the given minerals on the Mohs scale. - Corundum (C): 9 - Orthoclase (A): 6 - Fluorite (D): 4 - Calcite (B): 3

Step 2: Arrange these minerals in decreasing order of hardness. The order from hardest to softest is: 1. Corundum (9) 2. Orthoclase (6) 3. Fluorite (4) 4. Calcite (3)

Step 3: Write the final sequence using the letters. The sequence is C, A, D, B.