3.A.: Particles and waves
Particles are waves, waves are particles.
The wave corresponding to a particle is
.
In quantum mechanics the probability of finding a particle at a certain position at time t, P(x,y,z,t), is the square of the wave function.
Mathematically advanced students may note that a wave function may sometimes be a complex quantity, having both real and imaginary components. Then Equation (A) is generalized to read: