When initiating a paragraph, or a document, it is really important to take into account that the first sentences will not only capture the reader´s interest, but will also let him/her know about the topic, or theme, on which the document will be talking. As such, usually, unless the writer wants the reader to not glimpse the topic right away, the topic will be specified at the very first sentences within the opening paragraph. To do this, it is important for the writer to clarify what it is he/she will be talking about. In this case, the writer wants to talk about an attraction that is really impressive and unique, and which is completely natural; the hot springs. In order for the writer to make this clear to the reader, he must introduce the topic and then describe, or further explain, the reasons behind his/her argument, or main topic. From the choices given, then, the best one would be D: Mammoth hot springs are a popular attraction because the springs are unique, because this sentence lets us know what the whole paragraph will be about. If he started with the others, he would be telling us the descriptions of a hot spring, but we would not know that this is the topic to which the descriptions are refering.
