A、Inflexible. B、Diligent. C、Bulky. D、Bright. AWhich word can be used to modify the scientist?

In Europe, one can go from one’s home to an international conference with merely a long train ride in between. One scientist, after such a trip, came home looking ashen and distraught.
    "What happened?" ask his wife.
    "My dear," groaned the scientist, "I’ve been riding for hours in a backward-facing seat. You know how that always distresses my liver."
    "But, darling," said Iris wife, "we have talked of this before, mid surely you told me that if you found yourself in such a spot, you would ask the person in the seat facing you to exchange places."
    "Alas," said the scientist. "On this trip I could not ask. There was no one sitting there."