According to the passage, pushing factors include the following EXCEPT______.

A．are all related to economic considerations.
B．are not as decisive as push factors.
C．include a range of consideration.
D．are more important than push factors.

A．How biochemical changes occur in the human brain.
B．Why people should keep active not only physically but also mentally.
C．How intellectual activities influence brain-cell health.
D．Why people should receive special mental training as they ag