I could still have found my way into science in a nonexpansionist civilization of the future, where upward social mobility and its ______ opportunities would no longer be taken for granted to the degree that they have been in recent centuries.

A．It had something to do with the harsh environment our ancestors lived in.
B．It helped to conserve their resources and energy.
C．It could help them to survive the rough times.
D．It is impossible to find the indication of it in us nowadays.

A．Before he left Harvard.
B．On his return journey from Europe.
C．When he got married.
D．After his editor revised the manuscript.