题目

—Where is our English teacher? —She is in the classroom, _____ the exercises _________ the students have done at home． A．explaining;  /                                B．explaining;  for C．explained;  to                                D．explained;  from 答案：A如图所示，下面说法正确的是（   ）A、当S1,S2都断开时，L1,L2, L3串联   B、当S1,S2都闭合时，L1,L2, L3并联C、当S1,S2都闭合时，电路短路D、当S1,S2都闭合时，L1,L2, L3既有串联，又有并联，属混联