单项选择题
（）is not a physical wall but a flow virtual data to protect the computer network.
A.data wall B.data channel C.database D.firewall
单项选择题 （）means to provide facility for customer’s need or inquiry and arrange it.
单项选择题 Saving or reducing expenditure in business is（）
单项选择题 （）is guaranteed by the full market supply and just-in-time logistics.
