Identify the correct sentence. A. There's a lifeguard on duty it's safe to swim. B. There is a lifeguard on duty so it's safe to swim. C. There's a lifeguard on duty, it's safe to swim. D. When there's a lifeguard on duty, it's safe to swim.

Choice (D) is the most grammatically correct sentence. This sentence has a dependent clause followed by an independent clause, both separated by a comma. This allows for the two clauses to be properly joined and for the sentence to contain a logical flow.