Which one of the following is true ？

A、Onlyonemostsuitablemediumcanbeusedfornetworkconstruction
B、Networkdesignersarefreetochoosethemedia
C、Mediaprovidersneverparticipateintheselectionofnetworkmedia
D、Whenselectinganetworktransportmedia,thereisalwayssometrade-off（权衡）betweenperformanceandcost