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 am hesitating between 4 options to answer the following question, which option should I choose?
Question: What is the function of hollow bones in birds?
Possibilities:
- 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

to reduce the weight of birds for flight