Be careful ------- what you are doing.A with B in C of

答案：A
解释：be careful with 表示“小心,照顾”,是固定词组,比如：
Be careful with your luggage.
小心看好您的行李.
Be careful with this key, please? There's no spare.
留心保管这把钥匙,因为没有备用的.
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