【单选题】 The following statement about the functions of the OSI reference model layers is incorrect: ().
A.
The physical layer refers to the original bit stream transmitted over a communication channel. It defines the mechanical, electrical functions and procedures required to transmit data.
B.
The network layer decides the best route for transmitting messages. The key problem is to determine how to choose the route from the source end to the destination end of the packet.
C.
The basic function of the transport layer is to establish and maintain virtual circuits for error checking and flow control.
D.
The session layer is responsible for data format processing, data encryption, etc.