Four sentences related to a topic are given below. Three of them can be put together to form a meaningful paragraph. Select the odd one.

1. It is difficult to calculate the exact number of words.
2. If we calculate all of scientific nomenclature, this could easily double the figure.
3. Run on sentence is an ungrammatical construction in which two or more independent clauses are improperly joined without a conjunction.
4. For example, there are apparently one million insects already described in the scientific nomenclature with several million more awaiting description.