[高中] known for/ by?

[daffodilerin (星光)]

各位老師好...
我目前對於known for/ by的用法有點搞混
我知道known for是以～而聞名的意思
那know by呢？
以下是我遇到的題目
1. A man is known ______ the company he keeps.
   (A)for (B)by (C)to  (D)as
    Ans: (B) by

2. Mark Twain is a famous writer who is known ____ his sense of humor.
   (A)by (B)to (C)for (D)from
   Ans: （C）for

兩題的語意有不同嗎？？
感謝！！

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