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 negative
The correct answer is
A. (i) and (iii)
B. (ii) and (iii)
C. (i) and (iv)
D. (ii) and (iv)