Which statement about idle mode signaling reduction in the LTE network is true?

Which statement about idle mode signaling reduction in the LTE network is true?

A.
ISR support is not mandatory for UEs that support GERAN/UTRAN.

B.
ISR requires special functionality on the UE and the core network nodes (MME, SGSN,
and SGW).

C.
ISR requires special functionality on the UE and the core network nodes (MME and
SGSN).

D.
Gn/Gp SGSN supports idle mode signaling reduction functionality.