What was the major sign that lead Professor Shiller to predict the crash of the housing market?

A.Housing prices had been rising for 100 years, but then suddenly started to fall.

B.Housing prices had been rising at a steady rate for 100 years, but then suddenly started to rise at a much higher rate.

C.Housing prices had been falling for 100 years, but then suddenly started to rise.

D.Housing prices had been relatively constant for 100 years, but then suddenly started to rise.