题目内容：

___must come first and carefulness is the ___safe driving.
A,safe; key of B,being safe,key of
C,safe,key to D,driving safely,key to

优质解答

D.
driving safely 动名词短语做主语,safe形容词不做主语
key to　(key后面介词固定用to 表示“.的钥匙/关键所在”
整句翻译：安全驾驶第一,仔细是安全驾驶的关键所在.