The sentence A is the first line in the sequence as it introduces the subject of the paragraph ie our planet. This is also apparent from the fact that all the four options have A as the first sentence. C is the second sentence as it describes what is happening to the subject (it is getting smaller). B would be the third sentence, it describes what's being said about the subject. D would be the last sentence as it follows B in the sequence. (Some say... and others say.... are always listed in this order.)