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.

Particle exchange energy though elastic collision , hope this .

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant.