【单选题】The types of short-circuit in three-phase system are three-phase short-circuit, two-phase short-circuit, two-phase short-circuit grounding and single-phase short-circuit grounding. When the three phases of the system are short-circuited, they are still in the symmetrical stated. Other types of shorted circuit are asymmetric short circuit. Which of the following types is a symmetrical short circuit? A. Single phsae ground short circuit B. Three phase short circuit C. Two phase short circuit D. Two phase ground short circuit