Flow control is a function that prevents network congestion by ensuring that _（ ）_ do not overwhelm_（ ）_ with data．There are three commonly used methods for handling network congestion．_（ ）_is used by network devices to temporarily store bursts of excess data in memory until they can be processed．_（ ）_ are used by receiving devices to help prevent their buffers from overflowing．_（ ）_ 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．