Example of cargo heating system: identify correct labelling

A. 1-Steam Return Line; 2-Compressed Air Line ; 3- Steam Supply Line; 4- Toxic Gas Detector
B. 1-- Toxic Gas Detector; 2-Steam Return Line; 3- Steam Supply Line; 4-Compressed Air Line
C. 1-Compressed Air Line; 2-Steam Return Line; 3- Steam Supply Line; 4- Toxic Gas Detector
D. 1-Steam Supply Line; 2-Steam Return Line; 3-Compressed Air Line; 4- Toxic Gas Detector