In the given question, four statements are labelled as P, Q, R and S which when logically ordered form a coherent passage. Choose the option that represents the most logical order.

P: In order to draw air into its mouth, the frog lowers the floor of its mouth.

Q: This causes the throat to expand.

R: Then the nostrils open allowing air to enter the enlarged mouth.

S: The nostrils then close and the air in the mouth is forced into the lungs by contraction of the floor of the mouth.