i can not stop smoking docter for your health i am afraid yo

填 C ：have to
must 和have to 有以下区别：
1．强调重点不同：must 强调主观；而 have to 强调客观：
You must go now（I want you to go now）．你必须现在就走（有“我要你现在走”之意）。
If I buy that car,I'll have to borrow some money．如果我要买那辆汽车,我得借些钱。
2．时态形式不同：一般说来,must 只有现在时一种形式（在宾语从句中可表示过去）,而have to 则有多种时态形式：
He said（that）she must go with me．他说她一定要同我一起去。
She had to go there on foot yesterday．昨天她不得不步行去那儿。
He has had to buy a car．他不得不买了辆小汽车。
3．have to 可以有分词、动名词、不定式等非谓语形式,而 must则没有这些形式：
He only saw her in the evening,having to be away all day．他因整天得外出,所以只能晚上见她。
I regret to have to trouble you again．我很抱歉不得不再麻烦你一次。
4．否定式的意义不同：must 的否定式mustn't 意为“不准”、“不允许”；而 have to 的否定式 don't have to 意为“不必”=needn't）。