The population is increasing and _________________ we need more food.

The correct answer is 'therefore' which shows logical consequence between two independent clauses. 'The population is increasing and therefore we need more food' presents a cause-effect relationship. 'Therefore' is a conjunctive adverb that properly connects the idea that population growth creates a need for more food resources. Option A 'so' would need different sentence structure. Option C 'then' doesn't convey the logical consequence as precisely. Option D 'since' would indicate cause rather than effect.