Which of the following random variables would be discrete random variables?

$($Adapted from Milton & Arnold, $4$th edition, page $81)$

T: the turnaround time for a computer job $($time it takes to run the program and receive the results$)$

M: the number of meteorites hitting a satellite in a day day

$N$ : the number of neutrons expelled per thermal neutron absorbed in fission of uranium$-235$

O: the actual resistance in ohms for a $2\mathrm{.}5$ inch electric bell

P: the number of power failures in a month in the Tennessee Valley power network

$R$ : the lengths of great white sharks caught off the coast of Australia during a sixweek period

Only M

M$,$ N$,$ O$,$ P

$T,M,N,R$

Only $M$ and $P$

M$,$ N$,$ P