In this sentence, we would be understanding the subject-verb agreement. The rule states that when the plural noun denotes some specific quantity or amount of consideration as a whole, the verb is generally singular. In this case, two parts of the work are taken as a whole.

"is" is a singular verb. It is the correct answer.  

"were" and "are" are plural verbs. 

"have" is the root VERB and is generally used alongside the PRONOUNS I / You / We / Ye and They and PLURAL NOUNS. It is the wrong option

Option B is correct.