A customer called their wireless provider stating their $6-$year$-$old cell phone no longer connects to the network. The support rep informs them that the phone is no longer compatible with their network because they recently shut down their older cellular towers. What wireless technology might the sales rep be referring to$?$

Cellular signal

a$.$

Near$-$field communication

b$.$

$3$G

c$.$

Mobile hotspot

d$.$

Wi$-$Fi