对因瓦斯浓度超过规定被切断电源的电气设备，必须在瓦斯浓度降到（）以下时，方可通电开动。
参考答案：1.0％
21． More than one high official_____by the police since the mayor war shot dead.
A．has been questioned
B．have been questioned
C．was questioned
D．were questioned
为什么要求设计者在版图设计时必须遵循一定的设计规则？
