Answer:
Speed, 
Explanation:
The device which is used to accelerate charged particles to higher energies is called a cyclotron. It is based on the principle that the particle when placed in a magnetic field will possess a magnetic force. Just because of this Lorentz force it moves in a circular path.
Let m, q and V are the mass, charge and potential difference at which the particle is accelerated.
The work done by the particles is equal to the kinetic energy stored in it such that,

v is the speed with which the particles enter the cyclotron
So,

So, the speed with which the particles enter the cyclotron is
. Hence, this is the required solution.
I believe that the answer is A. Decaying
Argon is a suitable choice for light bulbs because it's inert. Compared to a reactive gas like oxygen, the metal filimant would burn up in a reactive enviroment, which is why a noble gas is used.