60.  A from time to time   B  in time    C  sooner or later   D on occasion

Small oil spills on deck can be kept from going overboard by（）.

Structural stress on a ship can be reduced by（）.

Stress on the topping lift of a swinging boom can be reduced by（）.

It should be apparent that little or nothing is gained by opening a drain at the same time as or after admitting steamThe above sentence probably means （）.

Microwave communication uses high-frequency(56)waves that travel in straightlines through the air. Because the waves cannot(57)with the curvature of the earth, they can be(58)only over short distance. Thus, microwave is a good(59)for sending data between buildings in a city or on a large college campus. For longer distances, the waves must be relayed by means of "dishes" or(60).These can be installed on towers, high buildings, and mountain tops.

Time can be saved by deciding on a format which suits you and then keeping a pile of blanks.（）

A bridge would cost far more than a tunnel, but you would be able to go by rail or by car on a bridge, whereas a tunnel would provide a rail link only.

The TCP protocolis a (1) layer protocol. Each connection connects two TCPs that may be just one physical network apart or located on opposite sides ofthe globe. In other words, each connection creates a (2) witha length that may be totally different from another path created by another connection. This means that TCP cannot use the same retransmission time for all connections. Selecting afixed retransnussion time for all connections can result in serious consequences. Ifthe retransmission time does not allow enough time for a (3) to reach the destination and an acknowledgment to reach the source, it can result in retransmission of segments that are still on the way. Conversely, if the retransnussion time is longer than necessary for a short path, it may result in delay for the application programs.Even for one single connection, the retransmission time should not be fixed.A connection may be able to send segments and receive (4)faster during nontraffic period than during congested periods. TCP uses the dynamic retransmission time,a transmission time is different for each connection and which may be changed during the same connection. Retransmission time can be made(5)  by basing it on the round-trip time (RTT). Several formulas are used for this purpose.

Excessive lube oil consumption by a reciprocating air compressor can be caused by （）.

The smallest size of flaw that can be detected on a radiograph examination of a weld will be indicated by the（）.

rogram propagates itself by modifying other programs to include a possibly changed copy of itself and that is executed when the infected program is(67). A virus often causes damage or annoyance and may be triggered by some event such as the occurrence of a predetermined date.Worm is a self-contained program that can propagate itself through data processing systems or computer networks. Worms are often designed by hackers to use up(68) resources such as storage space or processing time.Trojan horse implies by the name an apparently harmless program containing(69) logic that allows the unauthorized collection, falsification or destruction of data. Logic bomb causes damage to a data processing system when triggered by some specific system condition. Time bomb is also a malicious program being activated at a(70) time.