OG详解-OG8 文法 Q19

Choice A is the best answer. The convention being tested is finite verb use in a main clause. A main clause requires a finite verb to perform the action of the subject (in this case, Ashford’s “gestures” and “habit”), and this choice supplies the finite past tense verb “helped” to indicate what Ashford’s gestures and habit helped accomplish. 
Choice B is incorrect because the non-finite participle “helping” doesn’t supply the main clause with a finite verb. Choice C is incorrect because the relative clause “that helped” doesn’t supply the main clause with a finite verb. Choice D is incorrect because the non-finite to-infinitive “to help” doesn’t supply the main clause with a finite verb.