For stable structures, one of the important properties of flexibility and stiffness matrices is that the elements on the main diagonal________________?_x005F_x000D_i) of a stiffness matrix must be positive_x005F_x000D_ii) of a stiffness matrix must be negative_x005F_x000D_iii) of a flexibility matrix must be positive_x005F_x000D_iv) of a flexibility matrix must be negative_x005F_x000D_The correct answer is

• A. (i) and (iii)
• B. (ii) and (iii)
• C. (i) and (iv)
• D. (ii) and (iv)