在PCB设计中，下列哪个层是用于定义电路板上有效放置元件和布线的区域（）。

A:Signal layer B:Top layer C:Bottom layer D:Keep out layer

We in Japan {{U}}keep an eye out{{/U}} for our self-interest.

A:look at B:look up C:look back D:look out

（）is the 4th layer of the OSI reference model responsible for reliable network communication between end nodes.

A:Data link layer B:Session layer C:Transport layer D:Application layer

Which OSI layer does a Router operate at

A:the Physical Layer B:the Data Link Layer C:the Transfer Layer D:the Network Layer

Which OSI layer does a bridge operate at

A:the Physical Layer B:the Network Layer C:the Transport Layer D:the Data Link Layer

Which OSI layer does a Bridge operate at ( )

A:the Physical Layer B:the Data Link Layer C:the Transport Layer D:the Network Layer

Transport gateways connect byte streams in the transport layer. Application gateways allow interworking above

A:network layer B:data link layer C:transport layer D:application layer

Which OSI layer are both SLIP and PPP belonged to ( )

A:the Physical Layer B:the Network Layer C:the Data Link Layer D:the Transport Layer

Transport gateways connect byte streams in the transport layer. Application gateways allow interworking above

A:network layer B:data link layer C:transport layer D:application layer