柯林斯词典里标注的是可变名词。（注意最后一个例句）
They are prepared to hand back a hundred prisoners of war a day as a sign of good will.
他们准备每日遣返100名战俘以示友好。
His face and movements rarely betrayed a sign of nerves.
他的表情和行为几乎没有流露出紧张的迹象。
Your blood would have been checked for any sign of kidney failure.
应该要对你进行抽血检查的，看是否有任何肾衰竭的迹象。