Which system has the least effective radius of operation？A.HF SITORB.MF NBDPC.VHF DSCD.NAT

Which system has the least effective radius of operation？

A.HF SITOR

B.MF NBDP

C.VHF DSC

D.NATTEX

请帮忙给出正确答案和分析，谢谢！