An angle has a measure of 1 radian, when its vertex is placed at the center of a circle and has rays that... (select all that apply)
a. have an openness of 2pi.
b. wrap exactly half-way around the circle.
c. have a distance between them of 1 radius length.
d. are the length of 1 radius.
e. subtend (cut-off) an are length equal to 1 radius length.