challenge: What is the geographical feature that marks the southeastern corner of the Ordos Loop in Tongguan County?
  
  A. The intersection of the Qin Mountains and the North China Plain
  B. The point where the Qin Mountains turn the Yellow River sharply eastward
  C. The meeting of the Wei and Yellow Rivers with the Yangtze River
  D. The confluence of the Yangtze and Yellow Rivers
  Answer: 