Preventer bars are fitted on lifeboat releasing hooks to prevent（）.

A.the falls from unhooking if the releasing gear is operated accidentally while the boat is being lowered

B.operation of the release lever until the boat is waterborne

C.the falls from rehooking after they have been released

D.accidental unhooking when the falls become slack

在梁的整体稳定计算中，φ_b不等于______时，为梁弹性和弹塑性工作的分界点。