[单选题]Choose the closest paraphrased version after each of the following sentences or the italicized part. The grouping of these explosives is one of the most highly classified aspects of the atomic bomb. A The quality of the explosives matters a lot in classifying atomic bombs. B These explosives play the most important role in making the atomic bomb. C How to arrange these explosives is the top secret in making the atomic bomb. D What characteristics of these explosives are used in making the atomic bomb is still unknown.