The rope shown at an instant is carrying a wave travelling towards right, created by a source vibrating at a frequency n. Consider the following statements

I. The speed of the wave is $4\mathrm{n}\times \mathrm{ab}$
II. The medium at a will be in the same phase as d after  $\frac{4}{3\mathrm{n}} \mathrm{s}$
III. The phase difference between b and e is $\frac{3\mathrm{\pi }}{2}$
Which of these statements are correct