ACT English Practice Question

Read the following excerpt and consider the bolded portion. Which choice makes the sentence most grammatically acceptable? Later, sometime before 959 CE, she entered **the abbey at Gandersheim, it was** well known as a significant center of learning.

1. A.No Change
2. B.Gandersheim and, historically, this abbey was
3. C.Gandersheim, it was an abbey
4. D.Gandersheim, an abbey