（主要想请人分析句子结构再帮助解题）His elderly father must be getting thin,for his trousers,like a flag in the wind,________ badly.A.fly B.are flying C.have flown D.flew请分析句子结构!
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（主要想请人分析句子结构再帮助解题）His elderly father must be getting thin,for his trousers,like a flag in the wind,________ badly.A.fly B.are flying C.have flown D.flew请分析句子结构!
（主要想请人分析句子结构再帮助解题）
His elderly father must be getting thin,for his trousers,like a flag in the wind,________ badly.
A.fly B.are flying C.have flown D.flew
请分析句子结构!
（主要想请人分析句子结构再帮助解题）His elderly father must be getting thin,for his trousers,like a flag in the wind,________ badly.A.fly B.are flying C.have flown D.flew请分析句子结构!
选 B.are flying
句子意思是：他的老父亲一定是正在变瘦,因为他的裤腿儿就像风中的旗帜一样飞舞飘荡.
整句中分为主句和从句.
主句：His elderly father must be getting thin
原因状语从句：（for意思是因为,是原因状语从句的引导词）his trousers,like a flag in the wind,are flying badly.
主句中,主语：His elderly father ,谓语：must be getting,表语：thin
从句中,主语：his trousers,谓语：are flying,状语：badly,like a flag in the wind是插入语,修饰trousers,插入语的特征就是它不是句子必不可少的一部分,去掉也不影响句子的完整.
选B的理由：D是过去式,与整句时态不符；因为是在说裤子正在飘舞,要用进行时.
前半句可以不看了，后半句for 表原因，like插入语省略，
like a flag in the wind 是插入语，for后面是一个表示原因的句子，可翻译成“因为……”， 整个句子主干是His father must be getting thin, for his trousers are flying badly.