最近在看指考英文，综合测验有一题：
In certain situations such as in crowed subway cars or elevators, it is not al
ways possible for people to keep their _______ distance from others.
这一题答案是preferred，我有选对，但想请问在这里preferring和preferred的差异是什
么？
为什么不能用preferring?
是因为距离是“被偏好的”吗？
感谢