"Oxygen and hydrogen are gases."
Translation:Mae ocsigen a hydrogen yn nwyon.
This a typo which will be fixed in the future. Although h- is preceded by yr, it is not preceded by ac - although it is a common typo and minor error. The phrase should read:
- Mae ocsigen a hydrogen yn nwyon
While we wait for the ac version to be removed (which might take a while because of Duo's zombie sentence bug), the corrected a version will now appear from time to time as the question and it will always be accepted as the English to Welsh answer.