Reading Comprehension (RC) | Re: Most attempts by physicists to send particles faster

Could expert please help with the 2nd question? I went over the passage and the official answer several times but didn't figure it out.

Upon to my understanding - the tunneling time grows as the thickness of the barriers grow, until the tunneling time reaches his maximum ("the time it takes a particle to tunnel through barrier increases with the thickness of the barrier until tunneling time reaches a maximum"). Than the tunneling time doesn't affected by the thickness of the barrier