Which of the following is an indication of possible hacker activity involving voice communications?

A. A significant percentage of lines are busy during early morning and late afternoon hours.
B. Outbound calls are found to significantly increase in frequency during non-business hours.
C. Inbound calls experience significant fluctuations based on time-of-day and day-of-week.
D. Direct inward system access (DISA) is found to be disabled on the company's exchange.