What Makes It Difficult to Definitively Say a Certain Group

What makes it difficult to definitively say a certain group of people is a "terrorist group"?

A) Some acts of violence border on what is violent enough to be considered terrorism but may not fall under the scope of other terrorist activities.
B) Leaders of some terrorist groups also perform charitable acts, which makes it hard to consider their group one of terrorism.
C) Acts of violence considered terrorism might be seen from a different perspective by different groups of people.
D) All of these are correct.