13. Combine the following sentences into a coherent paragraph. (? )

In the fourth sentence of this passage, "just entered the grassland belt and forest belt" is a summary sentence of this paragraph. So let's describe the "grassland belt" first and then the "forest belt". (2) The sentence is about the grassland, while the sentence (135) is about the forest, so the sentence (2) should be ranked at the previous level, and the sentence (135) should be ranked at the next level. As for the internal ordering of the sentence 13⑤, the first sentence "on the south slope" and the second sentence "on the north slope" should naturally rank first; In the third sentence, "under the canopy of Chinese fir forest" is not only "before"-closely connected with the "original spruce forest" and "giant fir" in the first sentence, but also "after"-citing the activities of the following "wild animals" and "forest birds", so it should be ranked in the middle, and the remaining (5) sentences should be ranked behind. Looking at the above analysis, the order of several sentences should be 42 13⑤.