One of the items that follow is a fragment. Select the fragment.
a) During the chemistry examination on Tuesday morning, the room was silent and all of the students concentrated hard on the difficult problems Professor Mendoza had written.
b) During the chemistry examination on Tuesday morning, while the students concentrated hard on the difficult problems Professor Mendoza had written.
c) It was Tuesday morning, and the room was silent as the students concentrated hard on the difficult problems on the chemistry examination which Professor Mendoza had written.
d) The room was silent and all of the students concentrated hard on the difficult problems on the chemistry examination that Professor Mendoza had written.
