Certain insects, such as the monarch butterfly, can incorporate poisonous substances found in food plants into their own tissues and use them as ______.

A．Because engine trouble started.
B．Because the runway couldn’t be seen clearly.
C．Because the airport was not ready for the plane to land.
D．Because some traffic accident happened at the airport.

A．He needs a change.
B．To visit New Mexico.
C．Get into Albuquerque.
D．Go skiing.