单项选择题
A.number of turns of wire in the armature coil
B.number of magnetic poles
C.strength of the magnets used
D.output voltage
单项选择题 The pushbutton on the handset of a ships sound-powered telephone must be depressed to （）.
单项选择题 Which of the following statements is true concerning simple parallel resistance circuits?（）
单项选择题 Which of the listed items will stop a motor due to a reduction in voltage and restart it when the voltage is restored to normal?（）