A vacuum flask keeps its contents warmer or cooler than the surrounding environment for a length of time. It consists of two flasks, one inside the other, that connect at the neck. The space between the two flasks is completely empty of air. Why does the design of the vacuum flask minimize heat transfer by conduction?

A. One flask is smaller than the other flask.
B. One flask is more reflective than the other flask.
C. There is no air between the two flasks.
D. The two flasks do not touch each other, except at the neck.