OG详解-OG7 文法 Q8

Choice D is the best answer. The convention being tested is the use of punctuation within a sentence. This choice correctly uses a comma to mark the boundary between the main clause (“Recently...supposed”) and the supplementary element (“a finding...Paleolithic”) that provides additional information about the implications of the Neronian tool discovery. 
Choice A is incorrect because a semicolon can’t be used in this way to join the main clause (“Recently...supposed”) and the supplementary element (“a finding...Paleolithic”). Choice B is incorrect because it results in a rhetorically unacceptable sentence fragment beginning with “a finding.” Choice C is incorrect because it results in a run-on sentence. The main clause (“Recently...supposed”) and the supplementary element (“a finding...Paleolithic”) are fused without punctuation and/or a conjunction.