The topic sentence is the first and most important sentence in a paragraph.

A. 发电机提供三相电压。当发电机转子速度升高时，此电压的频率会增加。

B. 发电机提供三相电压。当发动机转子速度升高时，此频率的电压会增加。

C. 发电机提供三相电压。当发电机转子频率升高时，此电压的变化速度会增加。

A. 能够在紧急情况下改善操作安全性的要求和所推荐的空速(以及适用该空速的飞机配置)可被列在本节开头或列在《应急程序检查单》中。

B. 应在本节开头或《应急程序检查单》中列出，在紧急情况下能够避免事故的所需和建议的空速（以及适用于这些空速的飞机构型）。

C. 所需空速以及被认为可以在紧急情况下提高操作安全性的推荐空速(和对应的飞机构型)应在本节一开始就列出，或包含在《应急程序检查单》中。

A. Turn off all the unnecessary systems.

B. Disconnect all the systems which are not necessary for the test.

C. Disconnect all testing systems as necessary.

A. 每间隔20秒点击一次。

B. 咔哒声的时间间隔必须为20秒。

C. 必须将咔哒声的时间间隔调整为20秒。

A. Caution notes regarding possible hazards of using authorized and unauthorized cleaning agents shall be included.

B. Caution notes regarding use of unauthorized cleaning agents and possible hazards of using authorized ones shall be included.

C. Caution notes regarding use of authorized cleaning agents and possible hazards of use of unauthorized ones shall be included.

A. Move it lightly and make it through the hole

B. Move it slightly to make it aim at the hole

C. Move it gently to make it enter the hole

A. 一个生成器可以为所有系统提供电力。

B. 一个发电机可以为一台系统供电。

C. 一台发电机能够给所有系统供电。

A. The turbo engine includes a compressor bleed system, air cooling system, hot air system, and engine starting system. The compressor bleed system can control the airflow through the engine, and the air cooling and hot air systems are used for engine anti-icing.

B. The turbo engine includes a compressor exhaust system for controlling the airflow through the engine, a cooling air system for engine icing, and an engine starting system.

C. The turbine engines include compressor bleed systems used to control flow of air through the engine, cooling air systems, heated air systems for engine anti-icing, and engine starting systems.

A. The swivel nuts on both valves engage threads on an internal stem that loosens or draws tight the valve stem to a metal seat.

B. The rotating nuts on both valves are meshed with the threads on the inner valve stud, so that the valve stud is loosened or tightened to a metal valve base.

C. The swing nuts on both valves engage with the threads on the internal spool, allowing the spool to loosen or tighten to the metal seat.

A. Always use test element that can cause sparks.

B. Never use test equipment which can cause cracks.

C. Do not use test equipment that can cause sparks.