OG详解-OG1 文法 Q28

Choice B is the best answer. The sentence compares the lengths of the two rail tunnels, noting that the Channel Tunnel (about 31 miles long) is slightly shorter than the Seikan Tunnel (roughly 33 miles long).
Choice A is incorrect. The sentence makes a generalization about the length of some rail tunnels; it doesn’t compare the lengths of the two rail tunnels. Choice C is incorrect. The sentence describes a single rail tunnel; it doesn’t compare the lengths of the two rail tunnels. Choice D is incorrect. While the sentence mentions the two rail tunnels, it doesn’t compare their lengths.