is/are

[Lenka]

1)...one of the reasons IS / ARE climatic changes....

Which one is correct? Is or are?

2) Most of them ARE / IS there.

<= I can never recognize a subject in sentences like these... Is the subject one (most) or climatic changes/reasons (they)?

[Loctas]

Take this example:

One of them is Peter.

Most of them means Peter and some others.

So most of them are good guys.

[Lenka]

Take this example:

One of them is Peter.

Most of them means Peter and some others.

So most of them are good guys.

It means that I should use IS in ...one of the reasons IS climatic changes.... and ARE in Most of them ARE there?

[Loctas]

Yes