呼吸道包括鼻、咽、喉、气管和各级支气管。其中下呼吸道包括气管和（）。

For centuries animals have been used as surrogates for people in experiments to assess the effects of therapeutic and other agents that may be later used for humans.

A. which may later be used fr humans
B. that may be later used in humans
C. which might later be used for humans
D. that might later be used in humans