The correct answer is A: U.S. amateurs may find non-amateur stations in those segments, and must avoid interfering with them. When the Amateur Radio Service has secondary status in a frequency segment, amateur operators must yield to primary users (non-amateur stations) and avoid causing interference to them. Secondary status means that amateurs can use those frequencies, but they must not interfere with the primary users who have priority. Primary users are typically government, military, or other licensed services. Amateur operators must monitor before transmitting and immediately cease operations if a primary user appears. This is a fundamental principle of spectrum sharing - secondary users must be 'good neighbors' and yield to primary users.