我们班共有学生42名，每两人用一张课桌，需要多少张课桌？把这些课桌每4张摆成一行，能摆几行？还剩几张？

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.

（66）是（）

A. transmitting devices

B. transmitted devices

C. receiving devices

D. received devices