How does data collected in polls and studies lead us to incorrect conclusions?

Certification programs are offered by all of the following except

Two technicians are discussing how to test a car's exhaust by holding a small piece of paper within an inch of the tailpipe mouth. technician a says that if the paper is vibrating due to a "puffing" output from the tailpipe, this signals a smoothly running engine. technician b says that if the paper is being drawn toward the tailpipe, this could indicate burned exhaust valves. who is correct? a. technician b b. both technicians a and b c. technician a d. neither technicians a or b

Which of the following always exist between two objects having mass ● elastic force ● contact force ●gravitational force ●magnetic force