Sensors detect the concentration of mercaptan molecules in natural gas at various distances from a leaking source. at which location will a sensor detect the highest concentration of mercaptan? this is the answer but i need to know why this one. (one-half meter in front of the source)

I think it is one-half meter in front of the source because this is the nearest distance of the source and thus the sensor will register a higher concentration. 
At the nearest point the sensor will be able sense the high concentrations of the mercaptan which are nasty smelling sulfur compounds that are added to the natural gas piped to our houses.

13) a 14) a15) b16) i don't know sorry17) a (maybe)