A cable used to send data has an attenuation where power drops by a factor of 10 every 20 km. If the input power is 10 W, and the receiver can detect signals with a power of 10-4 W or more, what is the longest cable that can be used without a regenerator?
a. 80 km
b. 200 000 km
c. 100 km
d. 50 km