remians, remained or remains?

[Vaok]

when to have the next meeting and whether to invite the director still_____ a mystery to us.

A. remain B. remained C. remaining D. remains

[Torsten]

... still remains a mystery to us.
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[Vaok]

thank you.
why not A? Is it because "a mystery"?
what about this:
when to have the next meeting and whether to invite the director haven't been decided.

[Alan]

Hi,

Since the two sentences are joined with 'and', they tend to constitute a plural subject and would indicate 'remain' but I think the question is rather a tease because 'remains' would be possible also. If you turn it about, it becomes: It remains a mystery when ... and whether ...

I suppose I should plump for one or the other and suggest: remain.

Alan
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English as a Second Language
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[Vaok]

Alan, thank you very much.