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问题 选择题

关于摩擦力，以下说法正确的是（　　）

A．运动的物体不可能受到静摩擦力作用

B．静止的物体有可能受到滑动摩擦力作用

C．滑动摩擦力的方向总是与物体运动方向相反

D．滑动摩擦力的方向不可能与物体运动方向一致

答案

A、运动的物体受到的摩擦力不一定是滑动摩擦力，正在运行的传送带上的物体是运动的，但却受到静摩擦力．故A错误．

B、静止的物体受到的摩擦力不一定是静摩擦力，正在地面上滑行的物体，地面却受到的是滑动摩擦力．故B正确．

C、滑动摩擦力的方向与运动的方向可能相同，可能相反，与相对运动方向相反．故CD错误．

故选B．

单项选择题

选色时应首先确定（）

A.明度

B.彩度

C.色调

D.半透明区的分布及程度

E.个性特征
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单项选择题

When the system upon which a transport entity is running fails and subsequently reatarts, the ______ information of all active connections is lost. The affected connections become half-open, as the side that did not fail does not yet realize the promble.
The still active side of a half-open connections using a ______ timer. This timermeasures the time transport machine will continue to await an ______ of a transmitted segment after the segment has been retransmitted the maximum number of times. When the timer ______, the transport entity assumes that either the other transport entity or the intervening network has failed. As a result, the timer closes the connection, and signals an abnormal close to the TS user.
In the event that a transport entity fails and quickly restart, half-open connections can be teminated more quickly by the use of the RST segment. The failed side returns an RSTi to every segment i that it receives. When the RSTi reaches the other side, it must be checked for validity dased on the ______ number i, as the RST could be in response to an old segment .If the reset is valid, the transport entity performs an abnormal termination.

A. connection B. acknowledgment C. sequenceD. message
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