62 yo woman comes to the physician for a routine health maintenance exam. she feels well. Menarche was at the age of 13y, and menopause occurred 19y ago. she has no family hx of breast dz. examination shows a 2x2cm mass in the upper outer quadrant of the L breast. Mammography of the L breast shows calcifications in the upper outer quadrant. a biopsy of the breast mass shows invasive estrogen receptor-positive carcinoma with positive margins. which of the following is the most appropriate next step in management?
a. observation only.
b. cytotoxin and doxorubicin therapy.
c. tamoxifen therapy.
d. radiation therapy.
e. reexcision of the biopsy site.