On three separate occasions when she was out in public over the past three months, Vanessa has had to rush home after being overwhelmed by dizzy spells during which her heart pounded and she felt as if she couldn't breathe. On the last occasion she went to the emergency room, but a thorough examination ruled out any medical problem. Nevertheless, Vanessa has become increasingly fearful about having more episodes, and she has restricted her activities to only the most essential outings. Vanessa seems to be displaying symptoms of:

A. post-traumatic stress disorder.
B. generalized anxiety disorder.
C. social phobia.
D. agoraphobia.