Flow control is a function that prevents network congestion by ensuring that (66) do not over-whelm (67) with data. There are three commonly used methods for handling network congestion. (68) is used by network devices to temporarily store bursts of excess data in memory until they can be processed. (69) are used by receiving devices to help prevent their buffers from overflowing. (70) is a flow-control scheme in which the source device requires an acknowledgment from the destination after a certain number of packets have been transmitted.
（67）是（）
A. transmitting devices
B. transmitted devices
C. receiving devices
D. received devices
