Sentence
Our friends always spend their holidays in Italy. They started going there years ago.
.................................................................................................................. for years.

I have to complete the sentence following this example:
It's raining. The rain started two hours ago.
It'S BEEN RAINING for two hours.

It is an exercise about the present perfect continuous. In the same unit, the grammar book compare it with the present continuous focusing on the differences of use... I am not sure that in this specific case I have to use the present perfect continuous....

Question

Sentence
Our friends always spend their holidays in Italy. They started going there years ago.
.................................................................................................................. for years.

I have to complete the sentence following this example:
It's raining. The rain started two hours ago. 
It'S BEEN RAINING for two hours.

It is an exercise about the present perfect continuous. In the same unit, the grammar book compare it with the present continuous focusing on the differences of use... I am not sure that in this specific case I have to use the present perfect continuous....

Answer

Hello Chloe.

I think the present perfect continuous works well in this example.  I would say:

Our friends have been spending their holidays in Italy for years.

You would only use the present continuous if it was currently the holiday season and your friends were in Italy at this moment.  Then you could say:

Our friends are spending their holidays in Italy.

I am jealous of these friends!

Answer

It's in this context means "it has"

It HAS been raining for two hours...  Present continuous tense, it  means,  "it is still raining"
It HAD been ...                                       Past continuous tense..., it means, you are talking about a past action that is no longer valid in the present but you talk as if it is still happening.  You are recounting a past action at the present time.