21. 一雙筋矩形梁斷面，壓力筋深度d' =6.5cm，拉力筋深度d=50cm，若混凝土可提供之壓合力為85134kgf，壓力筋可提供之壓合力為46830kgf，Whitney混凝土等值應力區塊深度a=10cm，則下列何者最接近斷面之標稱彎矩強度？(A)58.7tf ●m (B)54.4tf● m (C)61.7tf ●m (D)57.5tf ●m