Vertical analysis is used to analyze financial statements by expressing each item as a percentage of a base amount. Which of the following statements best describes vertical analysis? 
a) Vertical analysis compares financial data across different companies. 
b) Vertical analysis expresses each item on a financial statement as a percentage of a base amount. 
c) Vertical analysis only focuses on income statements. 
d) Vertical analysis is primarily concerned with horizontal trends in financial data.