Hi Paul,

Your response to the following question


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1. 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
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was

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Hi hardworker, although I chose B for the same reason as Venksune, your questioning made me look for a black on white grammar rule which could explain this interesting concept. Interesting question indeed. I looked through bartleby.com and I figured that this question had to do with the use of "what". What I found was most interesting.

1- When "what" is the subject of a clause and the complement of the main clause are singular, the verb agrees with them
2- When "what" is the subject of a clause and the complement of the main clause are plural, the verb agrees with them

ie What he wanted after filing for divorce was the car, the house and the dog. --> you can see that the complement in blue are singular so "was" is singular. This is similar to the original question.
ie What he wanted after filing for divorce were the two cars, the properties and the pets. --> complement are plural so "were" is right.

3- Sometimes, you may find instances where "what" is not conjugated with the complement: This can happen in the case the writer wants to set a contrast between two nouns.
ie What John likes most are bread and butter --> this means that bread and butter should be considered as two independent nouns and each, seperately, accounts for what John likes
ie What John likes most is bread and butter --> this means that "bread and butter", as a single entity/group, is what John likes. Hence, "is" is appropriate.

4- When "what" is the object of a clause, if the complement is singular, then the verb should be singular.
5- When "what is the object of a clause, if the complement is plural, then the verb should be plural

Applying this to your examples, where "what" is used as object of a clause, you should get:
I see what seems to be a big tree. --> 3rd person singular is correct because the complement, big tree, is singular
I see what seem to be huge houses. --> 3rd person plural is correct because the complement, huge houses, is plural

You will find a link here which will give you more examples on the particularity of "what" as subject/object of a clause.
http://www.bartleby.com/61/93/W0109300.html
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Just one quick question would choice A be correct if it were the following ?

has been established by carbon-14 dating, but what are much more difficult to determine are

Thank you