用二只交流接触器控制电动机的正、反转控制电路，为防止电源短路，必须实现（）控制。
A.自锁
B.互锁
C.顺序
D.时间
参考答案：B
My parents had to sell the old house for my further study abroad even though it was
their own wishes.
A．on
B．against
C．for
D．above
请画出在X.25建议中定义的分组头格式。
