A stream of gas consists of n molecules. each molecule has mass m and speed v. the stream is reflected elastically from a rigid surface as shown (a "V" of molecules, with both arms of the "V" 30 degrees from the ground, 90 degrees for the center of the "V". the magnitude of the change in the total momentum of the stream is. 

a) 2mnv 

b) 2mnv sin 30 

c) mnv tan 30 

d) mnv cos 30 

e) mnv sin 30