The usual way to ensure reliable delivery is to provide the （71） with some feedback about what is happening at the other end of the line. Typically, the protocol calls for the receiver to send back special （72） frame bearing positive or negative （73） about the incoming frames. If the sender receives a positive acknowledgement about a frame, it knows the the frame has arrived safely. On the other hand, a negative acknowledgement means that something has gone wrong, and the frame must be transmitted again.An additional complication comes from the possibility that hardware troubles may cause a frame to （74） completely. In this case, the receiver will not react at all, since it has no any reason to react. It should be clear that a protocol in which the sender transmits a frame and then waits for an acknowledgement, positive or negative, will hang forever if a frame is ever lost due to, for example, （75） hardware.。

A．vanish

B．vary

C．appear

D．incline

经过建国后40多年的建设，我国以教育、科研、文化、卫生为主体的事业单位得到了蓬勃发展。据1992年末的初步统计，我国各类事业单位已经达到120万个，从业人员已发展到2533.5万人，约相当于全民所有制工业、建筑业企业职工总数的一半，占第三产业从业人员总数的30%左右。其中，直接隶属于中央国家机关的有3500个单位，从业人数240.8万，占事业单位总人数的 9.48%；省、自治区、直辖市党政群机关各部门所属事业单位21322个，从业人数244.9万，占总人数的9.67%，地、州、盟、市所属事业单位130186个，从业人数542.8万，占总人数的21.42%；县、旗、市所属事业单位341200个，从业人数740.2万，占总数的29.23%，乡镇事业单位703587个，从业人数764.8万，占总人数的30.20%。

1992年末，我国全民所有制工业、建筑业企业职工约为多少人（）

A．2533.5万人

B．5067.0万人

C．7600.5万人

D．无法确定