External gills, tracheae, and lungs all share which of the following sets of characteristics?

Options

A. Part of gas-exchange system, exchange both `CO_2` and `O_2`, increase surface area for diffusion.
B. Used by water breathers, based on countercurrent exchange, use negative pressure breathing.
C. Exchange only `O_2`, associated with a circulatory system, found in vertebrates.
D. Found in insects, employ positive-pressure pumping based on crosscurrent flow.

Answer

A