challenge: What was the primary goal of research by Christiane Nüsslein-Volhard and Eric Wieschaus in the 1970s?
  
  A. To understand how a simple egg can give rise to a complex segmented body plan
  B. To investigate the role of Hedgehog signaling in cancer
  C. To understand the genetic basis of human disease
  D. To study the developmental biology of plants
  Answer: 