What is the pulse duration, on/off time, and frequency for Iontophoresis?

a) 0.1-0.5 ms, 20-50 Hz, 20 minutes
b) 1-5 ms, 1-5 Hz, 30 minutes
c) 10-20 ms, 100-200 Hz, 15 minutes
d) 5-10 ms, 50-100 Hz, 10 minutes