고체역학 (Gere) — Stresses in Beams (Advanced Topics): Problem R-6.1

R-6.1 A composite beam is made up of a 200 mm $\times$ 300 mm core ($E_c = 14$ GPa) and an exterior cover sheet (300 mm $\times$ 12 mm, $E_e = 100$ GPa) on each side. Allowable stresses in core and exterior sheets are 9.5 MPa and 140 MPa, respectively. The ratio of the maximum permissible bending moment about the $z$ axis to that about the $y$ axis is most nearly: (A) 0.5 (B) 0.7 (C) 1.2 (D) 1.5