Which two sets of pins are looped on an RJ-45 56K loopback plug? (Choose two.)

A 56K loopback plug is designed to test the transmit and receive pairs of a CSU/DSU (Channel Service Unit/Data Service Unit) interface, which commonly uses an RJ-45/RJ-48C connector. In the standard T1/56K pinout for this connector, pins 1 and 2 are the receive (RX) pair, and pins 4 and 5 are the transmit (TX) pair. To create a physical loopback, the transmit signal must be routed back to the receive path. This is accomplished by connecting the transmit ring (pin 4) to the receive ring (pin 1) and the transmit tip (pin 5) to the receive tip (pin 2).

B. pins 1 and 7: This incorrectly pairs a receive pin (1) with a pin (7) that is typically unused or designated for cable shield ground in this standard.
D. pins 2 and 7: This incorrectly pairs a receive pin (2) with an unused or ground pin (7), failing to complete a valid loopback circuit.
E. pins 2 and 8: This incorrectly pairs a receive pin (2) with an unused or ground pin (8), which does not test the transmit/receive functionality.

1. Cisco Systems, Inc., "Loopback Tests for T1/56K Lines," Document ID: 12186. In the section "Hard Plug Loopback," the document explicitly states: "A T1/E1 loopback plug can be created by using an RJ-45 connector and a short Cat5 cable. Connect pin 1 to pin 4 and pin 2 to pin 5."

2. Cisco Systems, Inc., "Understanding T1 RJ-48C and RJ-45 Pinouts," Document ID: 112035. This document details the standard pinout for T1 interfaces using an RJ-48C jack, which is the basis for the loopback plug's wiring. It specifies: Pin 1 (Receive Ring), Pin 2 (Receive Tip), Pin 4 (Transmit Ring), and Pin 5 (Transmit Tip). This confirms that looping 1-to-4 and 2-to-5 connects the transmit pair to the receive pair.