Final answer:
The issue with the given expressions is the improper use of dot and cross product operations, which need to be corrected by applying the appropriate symbols and understanding that dot products yield scalars and cross products yield vectors. Therefore, the correct answer is b) Arrow notation is incorrect.
Step-by-step explanation:
The expressions given in question 21 are problematic because they mix vector and scalar quantities, and the operations are sometimes not correctly defined. For example, (a) should be C = A · B, indicating a dot product, and (b) should be C = A × B, indicating a cross product.
Correcting the expressions involves using the correct symbols and ensuring that the operation matches the type of product intended.
The dot product yields a scalar result, while the cross product yields a vector that is perpendicular to the plane containing the original vectors A and B as per right-hand rule. The dot product and cross product cannot be interchanged because they have different mathematical meanings and results.