[单项选择]Which of the following statements is true concerning all three-phase alternators?（）
A. Each has three separate but identical armature windings acted on by one system of rotating magnets
B. Each has one armature winding acted on by three identical but separate systems of rotating magnets
C. All three-phase alternators are designed to operate with a O8 leading power factor
D. The three phases always provide power to the load through three sets of slip rings and brushes

In sixteenth-century Italy and eighteenth-century France, waning prosperity and increasing social unrest led the ruling families to try to preserve their superiority by withdrawing from the lower and middle class behind barriers of etiquette. In a prosperous community, on the other hand, polite society soon absorbs the newly rich, and in England there has never been any shortage of books on etiquette for teaching them the manners appropriate to their new way of life.
Every code of etiquette has contained three elements, basic moral duties, practical rules which promote efficiency and artificial, optional graces such as formal compliments to, say, women on their beauty or superiors on their genero