Final answer:
In English grammar, the sentence: 'Sixty days ____ not enough time to complete the project', the correct form would be 'is'. This is because 'Sixty days', although sounds plural, is seen as a singular entity here, referring to a span of time to complete a project.
Step-by-step explanation:
In English grammar, verb agreement with the subject matters a lot. For the sentence: Sixty days ____ not enough time to complete the project, the correct form would be 'is'. Although 'Sixty days' sounds like a plural entity, in this instance it is considered a singular entity because it is referring to a specific amount of time taken as a whole to complete a project. So, we treat it as a singular subject and use singular verb form which is 'is'.
Learn more about Subject-Verb Agreement