How much force is needed to stop a 120-kg ice-hockey player if he decelerates at 20 m/s²?
a. 6 n
b. 240 n
c. 2400 n
d. 20 n

ANY force at all, acting in the right direction, will stop him. 
But smaller forces will take longer.

In order to either speed him up or slow him down at the rate
of 20 m/s every second, the force required is

           (mass) x (acceleration) = (120 kg) x (20 m/s²)

                                               = (2400 kg-m/s²) = 2400 N .

The force can either increase or decrease his speed at that rate,
depending on the direction in which it acts on him.

If the force is exerted opposite to the direction in which he's already
moving, then it can stop him if it continues long enough. 

What kind of enegia produces the mixture of bicarbonate and vinegar  chemical producetion

Sounds propagates through some kind of medium. usually it's air, but it goes just as well through solid objects like walls, windows etc. a soundwave will compress the (air)molecules in front of it, and if this increased density wave hits your ears, you register it as sound. solid objects like walls are a lot denser than air, this means that they will damp a wave, but also carry it easier.