Which of the following does not affect the period of a pendulum? O A. the length of the string
O B. the mass of the pendulum bob
C. the free-fall acceleration at the pendulum's location
D. All of the above affect the period.

i think the answer is a, hope this is right, that would be my best guess.

positive reinforcement? ?

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me