图示机构，以整体为对象，以下所列的平衡方程中，哪一个是正确的？A: åmO(F) = – m + P×b = 0 B:åmO(F) = – m + NA×b/cosq + ND×L = 0 C:åmO(F) = – m + P×b + ND×L = 0 D:åmO(F) = – m + P×b + NA× b/cosq + ND×L = 0 答案: åmO(F) = – m + P×b + ND×L = 0