OG详解-OG8 文法 Q16

Choice A is the best answer. The convention being tested is finite and nonfinite verb forms within a sentence. A main clause requires a finite verb to perform the action of the subject (in this case, “embryos”), and this choice supplies the clause with the finite present tense verb “enter” to indicate how the embryos achieve diapause.
Choice B is incorrect because the nonfinite to-infinitive “to enter” doesn’t supply the main clause with a finite verb. Choice C is incorrect because the nonfinite participle “having entered” doesn’t supply the main clause with a finite verb. Choice D is incorrect because the nonfinite participle “entering” doesn’t supply the main clause with a finite verb.