Two identical oscillators P and Q execute simple harmonic motion along the same straight line with equal amplitudes. For Q, the displacement is zero and increasing at t=0s and again at t=0.40s. For P, the displacement is zero and increasing at t=0.10s and again at t=0.50s. Taking ϕ=(phase of P)−(phase of Q), find the phase difference ϕ in radians (0<ϕ<2π).
(a) 43π
(b) 2π
(c) 23π
(d) 4π