A.
Stringers and the upper wing skin are called vent channels; The vent channels go through drain float valves in the center tank.
B.
Stringers and the upper wing skin are composed of the vent channels. The vent channels got exhaust float valves from the center tank.
C.
Stringers and the upper wing skin make the vent channels. The vent channels have drain float valves in the center tank.