The period when the great painted caves at Lascaux and Altamira were occupied by Upper Paleolithic people has been established by carbon-14 dating, but what is much more difficult to determine are the reason for their decoration, the use to which primitive people put the caves, and the meaning of the magnificently depicted animals.
(A) has been established by carbon-14 dating, but what is much more difficult to determine are
(B) has been established by carbon-14 dating, but what is much more difficult to determine is
(C) have been established by carbon-14 dating, but what is much more difficult to determine is
(D) have been established by carbon-14 dating, but what is much more difficult to determine are
(E) are established by carbon-14 dating, but that which is much more difficult to determine is

Learn the Grammar:
1. A clause is singular when used as the subject of a verb. So, the verb will be in singular form.

What I was trying to solve the problem is knowing the grammar.
=> This eliminates A, and D.

2. The period is singular. So, the verb of the period will be singular:

Example:
The period when X, Y, and Z happened IS

=> "When X, Y, Z happened" is the modifier of the period.

=> This eliminates C and E.

The remaining option B, which maintains the verb "has" for the subject period and uses the verb "is" for the clause, is the answer.

akhp77 wrote:Hello Guys,

Here, I would like to help you.

Above discussions are wrong.


Here, First subject is "The period" and the other subject is "what is much more difficult to determine".

Akhilesh,

It's good that you follow Ron just as we all do.Good keep it up !

But remember, before terming anything WRONG, do try to understand what the discussion is about. Just because a post doesn't look like Ron's conversation, doesn't meant that the post is incorrect.

Many of the above posters were confused and narrowed it down to options A and B. They were then unable to determine what should we use : is/are.

Stated in other words " what is much more difficult to determine is/are" .

Simple way is : read three words more in the original sentence. You'll encounter "the reason" . Yes! It is this noun which is difficult to find and it is singular! Hene, we will have to use is here.

Again, it's good that you are extending your help! But, an aspirin for a patient of heart-attack won't do much good.

Here, I do not have any intention to hurt ego of anybody. It is up to you; how to take it.

I have found that discussion was going in wrong direction. This is the reason that I said about Ron Lecture as I have already gone through. How to decide subject of a sentence.

I believe that there is nothing harm to share good things as required to solve issue.

akhp77 wrote:Here, I do not have any intention to hurt ego of anybody. It is up to you; how to take it.

really appreciate it!

akhp77 wrote: I believe that there is nothing harm to share good things as required to solve issue.

not at all! It's good!

But, as I said before, blindly terming anything as wrong is inappropriate. Case may be, you may be incorrect in your assumptions that ALL the posts above yours were WRONG. Hope you understand what I'm trying to say. It's good that you have your opinion. Everyone should have one!

If you want to add something to the discussion, you can just paste the link, saying that "here's more to it". You're done.

But, using capitalized letters in red color is something against netiquettes. Think about it.