Question

one of the rooms is or are open
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Answer 1

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The grammatically correct formation is —

One of the rooms was more spacious than the others.

I have added the last part so that the sentence becomes clear.

Explanation —

When we say one of certain things, we acknowledge that there are several things and the thing we are talking about is one among those things.

For example, I have 5 pens. 4 of them have blue ink and one has red ink.

I will say,

I have several pens. One of them/the pens has red.

The Verb also refers to the one and hence is Singular in Number.

Answer 2

Answer:

is

Explanation:

we are talking about one single room