答案：D
句意：汽车的声音消失在远方。
die down
1. (植物)枯萎，凋谢(但根仍可活)，(过冬时)假死，顶(梢枯)死，回枯：
The dahlias died down when we had the first cold spell.
第一次寒潮袭来时，大丽花的枝叶枯萎而假死了。
2. 逐渐平息，渐渐停止，逐渐消失，变弱：
The wind has died down quite a lot.
风小多了。
The fire is dying down.
火要熄了
die out
1. 灭绝，死光，不复存在：
Many species of animals are in danger of dying out.
许多动物种类都处于濒临灭绝的危险中。
2. 逐渐消失；渐渐停息：
Gradually the roar died out and the night became quiet.
吼声渐渐停息，夜晚变得寂静。
die off
相继死去，一个个死掉；绝种；顺次枯死；(声音等)逐渐消失：
The refugees were dying off from disease.
难民们相继因病丧身。
The species is dying off.
该物种面临绝种。
die away
1. (尤指声音、风、雨、光亮、势力、活动等)渐渐消失，逐渐消逝(或减弱，平息)，逐渐停止：
His anger died away.
他的怒气逐渐平息下来了。
The laughter died away.
笑声渐渐消失了。
2. 褪去；衰萎；渐渐凋落：
The rosy colour of her cheeks has died away.
她粉红色的双颊已渐衰萎。
3. 失去知觉；昏绝：
She died away at the sight.
她一看到这情况就昏过去了。
4. 【建筑学】衔接：
The arch died away against the towers.
弓形结构与塔状结构相衔接。
祝你学习进步，更上一层楼！(*^__^*)
追问请问die away和die down的区别，我混淆了。好像都有“消失，变弱”的意思。谢谢！
追答die away
1. (尤指声音、风、雨、光亮、势力、活动等)渐渐消失，逐渐消逝(或减弱，平息)，逐渐停止：
His anger died away.
他的怒气逐渐平息下来了。
The laughter died away.
笑声渐渐消失了。
2. 逐渐平息，渐渐停止，逐渐消失，变弱：
The wind has died down quite a lot.
风小多了。
The fire is dying down.
火要熄了
你多多看看这个两个例句，其实他们的区别就应该知道了。你是聪明人，你懂的！