（1）开始运动后，经多长时间质点的速度刚好为零？ （2）从开始运动到t0=6s末

一个凸多边形，除一个内角外，其余（n-1）个内角的和为2400°，则n的值是______．

在（），我国停止发行国债。

A.20世纪60年代和70年代

B.20世纪50年代和60年代

C.20世纪50年代和70年代

D.20世纪70年代和80年代

就业是指具有（）和（）的公民，在事实上获得有相应报酬的职业。

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
根据上述信用证，回答下列问题

信用证号码

电气工程中常用的合成纤维丝有（）和聚酯纤维丝。