The following sentence contains an introductory phrase:
As he listened to the heavy footsteps circling his tent Antonio regretted his
decision to go camping alone.
Which revision correctly punctuates the sentence?
a) As he listened to the heavy footsteps circling his tent Antonio regretted his
decision to go camping, alone.
b) As he listened to the heavy footsteps circling his tent, Antonio regretted his
decision to go camping alone.
c) As he listened to the heavy footsteps, circling his tent Antonio regretted his
decision to go camping alone.
d) As he listened, to the heavy footsteps circling his tent Antonio regretted his
decision to go camping alone.