Consider the following NPs:
500 years of history
100 liters of milk
300 grams of apple pie
When used as subjects in full clauses, which verb form do these NPs take – singular or plural verb? Am I right in thinking one could argue for either verb form here, depending on whether one sees years or history/liters or milk/grams or apple pie as the head of the NP?
That is, am I right in thinking that if we consider years to be the head of the noun phrase 500 years of history, a subsequent verb should be plural (e.g., 500 years of history are studied in this class), whereas if we consider history to be the head, a subsequent verb should be singular (e.g., 500 years of history is studied in this class), or should the verb always be singular (or plural) in this kind of construction?