question: I need you to determine the most frequently occurring letter in the dataset column 'CountryName' that we use in our econometric model. Please provide your response in the following format:
  
  - Format: Choose one from options: ('A','E','I','O','U')
  - Format: Number of bullet lists: 1
  - Keywords: letter frequency
answer: - Format: Choose one from options: ('A','E','I','O','U')  
  - Keywords: letter frequency