Two progressive waves of the same frequency travel in the same direction through air. Their amplitudes are 3.0cm and 4.0cm respectively. At a certain point, the amplitude of the resultant wave is observed to be 5.0cm. The phase difference between the two waves at this point is closest to
(a) 120∘
(b) 90∘
(c) 60∘
(d) 150∘