For stable structures, one of the important properties of flexibility and stiffness matrices is that the elements on the main diagonal (i) Of a stiffness matrix must be positive (ii) Of a stiffness matrix must be negative (iii) Of a flexibility matrix must be positive (iv) Of a flexibility matrix must be negativeThe correct answer is

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