challenge: What event in 1971 and 1972 led to a shift in Gilbert Hegemier's professional focus toward earthquake safety?
  
  A. The 1989 Loma Prieta earthquake and the 1994 Los Angeles earthquake
  B. The 1964 Alaska earthquake and the 1965 Chile earthquake
  C. The 2004 Sumatran earthquake and the 2005 Kashmir earthquake
  D. The 1971 San Fernando Valley and 1972 Nicaragua earthquakes
  Answer: 