In the following question, there are six sentences marked S1, S6, P, Q, R, S. The positions of S1 and S6 are fixed. You are required to choose one of the four alternatives which would be the most logical sequence of the sentence in the passage.

S1. While crossing a busy road we should obey the policeman on duty.

P: We should always cross the road at the zebra crossing.

Q: We must look to the signal lights and cross the road only when the road is clear.

R: If there are no signal lights at the crossing, we should look to the right, then to the left and again to the right before crossing the road.

S: If the road is not clear we should wait.

S6. We should never run while crossing a road.