问题
选择题
编号为1、2、3、4、5、6、7的七盏路灯，晚上用时只亮三盏灯，且任意两盏亮灯不相邻，则不同的开灯方案有（ ）
A．60
B．20种
C．10种
D．8种
答案
根据题意，先安排4盏不亮的路灯，有1种情况，
排好后，有5个空位，
在5个空位中任选3个，插入3盏亮的路灯，有C53=10种情况，
则不同的开灯方案有1×10=10种；
故选C．
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编号为1、2、3、4、5、6、7的七盏路灯，晚上用时只亮三盏灯，且任意两盏亮灯不相邻，则不同的开灯方案有（ ）
A．60
B．20种
C．10种
D．8种
根据题意，先安排4盏不亮的路灯，有1种情况，
排好后，有5个空位，
在5个空位中任选3个，插入3盏亮的路灯，有C53=10种情况，
则不同的开灯方案有1×10=10种；
故选C．
| 完形填空。 | ||||
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