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)

Ans: A