Which of the following examples shows degradation of word meaning?

Which of the following examples shows degradation of word meaning?

A.silly meaning from "happy" to "foolish"

B.accident meaning from "event"to"unfortunate event"

C.knight meaning from "servant"to"rank below baronet"

D.mill meaning from"place for grinding grain into flour"to "place where things are mde

正确答案是A