Question #N1174

The study found that in the early phases of the universe, the rate of expansion was much higher than it is now. It is known as the inflationary period. During that period, the universe expanded to the size that it is today, in a very short time. This period is followed by a slower rate of expansion, which is still occurring today.

Which choice best combines the sentences at the underlined portion to create a grammatically correct and clear sentence?
A. It is known as the inflationary period; during that period, the universe expanded to the size that it is today, in a very short time.
B. It is known as the inflationary period, during which the universe expanded to the size that it is today, in a very short time.
C. It is known as the inflationary period: during that period, the universe expanded to the size that it is today, in a very short time.
D. It is known as the inflationary period, and during that period, the universe expanded to the size that it is today, in a very short time.