The statement "The decline leveled off four years ago" (Last line, paragraph 2) implies that ______.

A．the number of children at boarding schools dropped sharply tour years ago
B．many boarding schools had to close four years ago
C．the changing of the number of the pupils in the boarding school became level after a sharp declining four years ago
D．more and more parents tended to send their children to some private school other than the boarding ones