72. IP networks are often described as “best-effort” networks. Which one of the following descriptions about “best-effort” is correct？ (A) “Best-effort” implies reliable delivery. (B) “Best-effort-delivery” does not provide any guarantee about quality of service. (C) “Best-effort” means secure delivery where all packets are encrypted when transmission. (D) “Best-effort-delivery” guarantees that all packets are received in order.