Which One of the Following Sentences Has a Missing or Unclear

Which one of the following sentences has a missing or unclear antecedent?

A) Every winter Danella and Rebecca take skiing lessons, but Rebecca broke her leg this year.
B) The teacher told the student that the student should go to college.
C) They say that it is better to keep one's mouth shut than to open it and sound like a fool.
D) The local radio station announced the winner.