女性，32岁，某温度计厂烘表工，近4年来常感头痛、头昏、心烦、易怒、记忆力明显下降，近1年来出现手指震颤，在做精细活时尤为明显。其最可能接触的职业性有害因素是（）

A.铅

B.苯

C.锰

D.汞

E.锌

The usual way to ensure reliable delivery is to provide the (16) 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 (17) frame bearing positive or negative (18) about the incoming frames. If the sender receives a positive acknowledgement about a frame, it knows 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 (19) 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, (20) hardware.

（18）是（）

A. application

B. connection

C. stream

D. acknowledgement