Explanation
Correct answer: D.
Choice D is the best answer. The independent clauses "researcher Lin Zhi…silk" and "by dissolving…2 GPa" can be grammatically separated by a period. They can stand alone as sentences, and this is the only choice that lets them do that.
Why the other choices are wrong
Choice A
Choice A is incorrect. This choice results in a grammar error called a comma splice. "Researcher Lin Zhi…silk" and "by dissolving…2 GPa" are both independent clauses. They need to either be separated with punctuation like a period or a semicolon, or they need to be connected by a comma and a coordinating conjunction like "and." A comma alone isn’t enough.
Choice B
Choice B is incorrect. This choice results in a grammar error called a run-on sentence. "Researcher Lin Zhi…silk" and "by dissolving…2 GPa" are both independent clauses. They need to either be separated with punctuation like a period or a semicolon, or they need to be connected by a comma and a coordinating conjunction like "and."
Choice C
Choice C is incorrect. This choice results in a grammar error called a run-on sentence. "Researcher Lin Zhi…silk" and "by dissolving…2 GPa" are both independent clauses. The coordinating conjunction "and" isn’t enough to link them by itself. We need a comma, too.