Sentence 1: A caterpillar was crawling along a wall. 
 Sentence 3: He observed it fall from the wall onto the ground 
 Sentence 4:  Feeling pity, the student took a leaf and picked up the caterpillar 
 Sentence 5:  He placed the caterpillar on a nearby tree 
 Option 1: A nearby student was walking from class when he saw the caterpillar. 
 Option 2: A student walked by and knocked the caterpillar off the wall.