The purpose of the skirt or trunk in four-stroke cycle engines is to act in a similar manner to a （）.

为保证电子商务系统的安全，电子商务身份认证的方式包括（）

Hi! Jim. This is my brother Peter.（）

 ______ twice a year, whether it is a car or a bus or a truck, is the rule that every driver must obey in this city.

Scientists say it may be five or six years _____ this medicine is tested on human beings.

The goal, which they are unlikely to live to see , is tocure, prevent or manage all diseasesin the next 80or so years.

Every deliberate act of wrongdoing by the master or any of the crew against the ship or cargo，without the authorization or privity of the Shipowner or his agent，is（）.

（）, or drum is used for hauling in or letting out the wires or ropes which will fasten the ship to the shore.

The （） is used for hauling in or letting out the wires or ropes.

It is over two years since my brother ______the army.

信息安全是电子商务安全问题的最终形式。

电子商务的安全主要是实现电子商务的保密性、（）、（）、不可伪造性和（）。