What primarily determines airway resistance in the respiratory passageways?

Their diameters  

Explanation:

An equation for the resistance to flow of a fluid through a pipe is

where

η = the viscosity of the fluid

l = the length of the pipe  

r = the radius of the pipe

The fourth-power term shows that even small changes in the radius (or diameter) of the passageways will cause large changes in airway resistance.

When the diameter of the airway gets as small as 1 mm, the resistance to air flow is quite large.

The flowers are breathtaking.

Explanation:

The leaves are green is an observation. The plant producing oxygen is a scientific fact. The bush having thorns is also an observation.

On the other hand, the flowers being breathtaking is simply a matter of opinion. So, it is an inference.

Hope this helps!