From 1780 to 2010, the amount of land defined as natural habitat decreased worldwide. However, from 1960 to 2010, this amount has remained relatively stable. Which of the following statements best summarizes the trend of natural habitat land over the specified periods?
a) It steadily increased from 1780 to 2010.
b) It sharply decreased from 1960 to 2010.
c) It fluctuated unpredictably throughout the entire period.
d) It decreased overall but stabilized from 1960 to 2010