A 20- year- old college freshman is brought to the attention of the dormitory resident assistant b/c she is not attending meals in the dining hall. The student says she feels anxious about being in the crowded kitchen and dining hall. Similarly, she has ordered all her schoolbooks online to avoid being in the bookstore b/c "it is too overwhelming". She also has not attended some of her crowded lectures. The student is able to have a good conversation in the hall of the dormitory with the resident assistant. The student denies fearing scrutiny by others. She explains that she avoids some places b/c she found that in malls or other crowded ares, she feels very worried, faints, sweaty, and dizzy. She is afraid of losing control and in being in situations where she cannot escape. What is the likely diagnosis?

A. Acute stress disorder.
B. Agoraphobia.
C. Posttraumatic stress disorder.
D. Social anxiety disorder.