单项选择题 The switchboards for a-csystems differ from the d-cswitch gear in that the open type panels are generally（） and the dead-front switch gear is the common rule.

单项选择题 Electrical supply is always needed even if only for lighting so after a ship goes into service it is extremely rare for the main bus-bars to be dead, it therefore follows that the bus-bars are continuously alive（） the life of the ship.

单项选择题 The current flowing first in one direction and then in another is called（） current.