The exact year when the Mogao caves in Dunhuang, China began to be sculpted and chiseled by the Northern Wei Dynasty has been difficult to verify, but what is much easier to determine are the purpose of their existence, the religious significance that the drawings on the walls of the caves held, and the order in which the caves were constructed.


(A) has been difficult to verify, but what is much easier to determine are

(B) has been difficult to verify, but what is much easier to determine is

(C) have been difficult to verify, but what is much easier to determine is

(D) have been difficult to verify, but what is much easier to determine are

(E) are difficult to verify, but that which is much easier to determine is


[spoiler]The contenders are a and B. However I took the compound subject. "the purpose of their existence, the religious significance that the drawings on the walls of the caves held, and the order in which the caves were constructed" to be plural, hence I chose A.[/spoiler]

OA-B