The given schematic illustration shows three steps 'P', 'Q' and 'R' of the polymerase chain reaction. Which of the following statements are correct with reference to the illustration given above ?

(i) Step is showing denaturation at low temperature.

(ii) Step is a denaturation of DNA strand at high temperature, followed by annealing.

(iii) Step is an extension of DNA in presence of thermostable DNA polymerase.

(iv) Step is extension with two sets of primers.