参考答案：A

解析：[分析] 习语辨析。 back out不履行(协议、承诺等)：It's too late to back out of the deal now．现在要在这笔交易上打退堂鼓为时已晚。back down让步，放弃，屈服：He proved that he was right and his critics had to back down，他证明了他是正确的，而那些非难他的人不得不收起他们那一套。back up使聚集：Traffic backed up in the tunnel．在地下道中引起了交通阻塞；back away因害怕或厌恶而后退：The child backed away from the big dog．那个孩子看见大狗就向后退了。