Introduction: In structural engineering and mechanics, matrices are used to represent the flexibility and stiffness of structures. These matrices play a crucial role in analyzing the behavior of structures under different loading conditions. Understanding the properties of these matrices is fundamental to ensuring the stability of structures. This question delves into the characteristics of the elements on the main diagonal of flexibility and stiffness matrices.

Question: What is an important property regarding the elements on the main diagonal of flexibility and stiffness matrices for stable structures?

A. Elements on the main diagonal of a stiffness matrix must be positive.

B. Elements on the main diagonal of a stiffness matrix must be negative.

C. Elements on the main diagonal of a flexibility matrix must be positive.

D. Elements on the main diagonal of a flexibility matrix must be negative.