A steel plate d × b is sandwiched rigidly between two timber joists each D × B/2 in section. The steel will be (where Young’s modulus of steel is m times that of the timber)_________________?
A. BD² + mbd²)/6D]
B. BD3 + mbd3)/6D]
C. BD² + mbd3)/4D]
D. BD² + mbd²)/4D]