Five sentences related to a topic are given below. Four of them can be put together to form a meaningful and coherent short paragraph. Identify the odd one out. Choose its number as your answer and key it in.

1. People who study children's language spend a lot of time watching how babies react to the speech they hear around them.

2. They make films of adults and babies interacting, and examine them very carefully to see whether the babies show any signs of understanding what the adults say.

3. They believe that babies begin to react to language from the very moment they are born.

4. Sometimes the signs are very subtle - slight movements of the baby's eyes or the head or the hands.

5. You'd never notice them if you were just sitting with the child, but by watching a recording over and over, you can spot them.