Learn English – “They are not” OR “It is not”

There is a tendency for the plurals of Latin words to be treated over time as singular in English and eventually to lose their singular forms, changing their meanings in doing so. Agenda, stamina and data are three examples. This doesn’t seem to happen as much with Greek words such as criterion/a. The reason is possibly that such words are more learnèd and less frequent and may be used by people who know and insist on the difference between the singular and plural. However, as far as phenomena is concerned, ‘The Cambridge Guide to English Usage’ reports that there are instances of its singular use as early as the sixteenth century and that there is good corpus evidence that is gaining ground now. Research in Australia in the 1970s, the article continues, showed that most young people there thought of the word as singular. The article concludes that ‘phenomena seems to be consolidating its position for plural uses, apart from extending its influence into the singular' (my emphasis).

You should use singular in this case.

There is only one units digit in each multiple, and it is 0.

Much the same can be accomplished in any complex network:

• Everyone in our branch offices has an accountant as a supervisor.
• All the people in our branch offices have an accountant as a supervisor.

The difference between the singular has and plural have is because every is singular, while all is plural (I don't make up these rules). But there is only one supervisor in each case, and it's an accountant who's the supervisor.