The following six sentences come from a paragraph. The first and the last sentences are given. Choose the order in which the four sentences (P, Q, R, S) should appear to complete the paragraph.
S1. Once an ant saw, hanging from a plant, a mature chrysalis which could just move its tail.
S2. __________________________________________________________
S3. __________________________________________________________
S4. __________________________________________________________
S5. __________________________________________________________
S6. The next moment the butterfly rose in the air and was soon out of the ant's sight.