to streamline birds for efficient flight to reduce the weight of birds for flight to store oxygen for high-altitude flight to make bones strong enough for flight
I gave my students this multiple choice question: What is the function of hollow bones in birds?

Only one answer is correct among these 4 choices:
- to streamline birds for efficient flight
- to reduce the weight of birds for flight
- to store oxygen for high-altitude flight
- to make bones strong enough for flight

Could you tell me which one is correct?

to reduce the weight of birds for flight