The correct answer is D. A<span>nimal testing is a cruel way to advance the progress of science. A simple sentence is one which has one predicate, or finite verb in it. In sentence D, the only predicate is IS, whereas <em>to advance </em>is infinitive, and thus cannot be considered a predicate. The other sentences have more than one predicate, and therefore are not simple sentences, but rather either complex or compound.
