设置操作限制开关，限制不应该发生的冲击及运动的预设制动装置，设置把手和预防下落的装置，失效安全的限电开关等。这些装置应用了()。P168

A.机械失效安全

B.机械设计本质安全

C.机械部件的定位安全

D.机器的安全布置

The ABC Bank Irrevocable documentary credit Number: LC123-258866 Date: August 24,1999 Date and place of expiry : October 30, 1999 China Advising bank: Bank of China Beneficiary: China XYZ import and export corp. Applicant: UVW corporation. Amount: USD52,500 (Say U.S Dollars fifty two thousand five hundred only) We hereby issue irrevocable documentary credit which is available with: Any bank By negotiation of beneficiary’s drafts at sight For full invoice value Drawn on the ABC bank Partial shipments: Not allowed Transshipment: Not allowed Shipment from: Shanghai To: Tokyo Japan At the latest: October 15, 1999 Following documents required: +Signed commercial invoice in three copies +Full set of clean on board ocean bill of lading +Weight certificate in two copies Covering: 100 metric tons (5PCT MOL) of XY chemical in drums at USD 500 per M/T. Special instructions: Insurance to be effected by buyer. Instructions to advising bank: Please advise this credit to the beneficiary We hereby undertake that drawers, endorsers in compliance with the terms of this credit will be duly honored upon presentation to the drawee bank. Authorized signature 一、根据上述信用证，回答下列问题

信用证的最高金额