We tend to realize the value of ______ (我们所拥有的东西) until there is no more of it in our life.

A．In order to elevate their nests.
B．In order to live more comfortably.
C．In order to avoid predators.
D．In order to make incubation easier.

A．Teachers like Professor Johnson are rare.
B．Professor Johnson has won a million dollars.
C．Professor Johnson is likely to be teaching at that school.
D．There are many teachers as good as Professor Johnson.