(A) should affect the both groups equally, so it’s not an alternate hypothesis. Also, a close resemblance to the natural scent could strengthen by defending from an objection that the scents were too different to reliably test sense of smell.

This shows that the test wasn’t investigating permanent damage. If the sulfur group was tested in the sulfur-emitting factory, whatever effect sulfur had on the group could have been due to the contemporaneous effects of sulfur. We’d want the test to be done outside the factory.

This could provide an alternate explanation for why the control group was able to identify more scents than the sulfur group. Prior experience with studies identifying scents could have improved their ability to identify scents in such tests.

This could provide an alternate explanation for why the sulfur-emitting factory group identified fewer scents. Maybe any damage to sense of smell was due to a different fume besides sulfur.

This could provide an alternate explanation for why the sulfur-emitting factory group identified fewer scents. Less familiarity with the natural scents they were supposed to identify could have accounted for their worse performance on the test.