Identify the sentence that uses dashes correctly to add clarifying information.
a) The biology course requires a good deal of reading, but I will enjoy it—I'm not working this semester.
b) My roommate—we've shared a place for two years—was surprised that his health and physical education course had a textbook.
c) The statistics course I took last year was incredibly boring—it was a summer course that was eight weeks long.
d) The Victorian British literature course has a heavy reading load—one novel a week—but an engaging professor.