jCorona usually occurs when the electrostatic stress in air around the conductor exceeds?

by

jCorona usually occurs when the electrostatic stress in air around the conductor exceeds?

A. 6.6 kV (r.m.s. value)/cm
B. 11 kV (r.m.s. value)/cm
C. 22 kV (maximum value)/cm
D. 30 kV (maximum value)/cm