Which sentence is punctuated correctly?
A. Luckily, we were able to spend a few hours swimming before the storm began.
B. Luckily we were able to spend a few hours swimming, before the storm began.
C. Luckily we were able, to spend a few hours swimming, before the storm began.
D. Luckily, we were able to spend a few hours swimming, before the storm began.
