A 28-year-old woman is undergoing an annual physical examination. An enlarged uterus was found on bimanual examination. No adnexal mass are noted and HCG (-). Rectovaginal examination is unremarkable. Which of the following conditions is the most likely cause of the enlaged uterus.? ( ) A: Pregnancy. B: Leiomyoma. C: Adenomyosis. D: Endometriosis. E: Carcinoma.