单项选择题UsingFTPpassivemode,aftertheclientopensthecommandchannel(port21)totheFTPserverandrequestspassivemode,whatwillbethenextstep?()
A. The FTP server sends back an acknowledgment (ACK) to the client
B. The FTP server allocates a port touse for the data channel and transmit that port number to the client
C. The FTP server opens the data channel to the client using the port number indicated by the client
D. The FTP client opens the data channel to the FTP server on Port 20
E. The FTP client opens the datachannel to the FTP server on Port 21