Usage of 'is' and 'are'...Ron/Mitch/Stacie/Bill?

[thestartupguy]

Sentence A: What concern me are the election results

Sentence B: The time when the apartment building in downtown Milwaukee started on fire has been determined by the fire department, but what is more problematic to establish is the source of the flame, the mechanism by which it ignited the building, and the extent of the damage

Sentence C: Around the corner, there are a bakery and a supermarket.

The above sentences are correct. I, however, marked the option which says what is more problematic to establish are the source of the flame, the mechanism by which it ignited.. considering that it were a list - the source of the flame, the mechanism by which it ignited the building, and the extent of the damage and hence plural as in sentence C.

1) Where did i go wrong?
2) When do we use 'is' and when do we use 'are'? Please explain with some more examples.

[avik.ch]

Sentence A: What concern me are the election results

This is not a GMAT kind sentence as GMAT dont use "me" - 1st person pronoun. Infact subject verb agreement is quiet different in this type of construction.

We can ignore this example. But the point relevant to GMAT SC - all noun clauses are singular.

What concern jim is the election result.

Sentence B: The time when the apartment building in downtown Milwaukee started on fire has been determined by the fire department, but what is more problematic to establish is the source of the flame, the mechanism by which it ignited the building, and the extent of the damage

here again "what is more problematic..." is the noun clause. As all noun clauses are treated as singualar - "is" is prefect.

Sentence C: Around the corner, there are a bakery and a supermarket.

this is the case of extraposition. here the subject is "a bakery and supermarket", which is plural and hence "are" is correct.

Flowers were everywhere. - original sentence. -- > extraposed one
There were flowers everywhere. - correct.


Hope this helps !!